eiouxnr 

LIBRARY 
G 


ESSENTIALS 


OF 


HUMAN  PHYSIOLOGY 


BY 

D.  NOEL  P^TON,  M.D.,  B.Sc.,  F.R.C.P.  ED. 

SUPERINTENDENT  OP  THE  RESEARCH  LABORATORY  OF  THE  ROYAL  COLLEGE  OF  PHYSICIANS 

OF  EDINBURGH;  LECTURER  ON  PHYSIOLOGY,  SCHOOL  OF  MEDICINE  OF  THE  ROYAL 

COLLEGES,  EDINBURGH  ;  EXAMINER  IN  PHYSIOLOGY  IN  THE  UNIVERSITY 

OF  GLASGOW  AND  FOR  THE  ROYAL  COLLEGE  OF  PHYSICIANS, 

EDINBURGH  ;   AND  LATE  EXAMINER  IN  THE 

UNIVERSITY  OF  EDINBURGH 


SECOND  EDITION 

REVISED  AND   ENLARGED 


or  THE 


SSITY 

WILLIAM    GREEN    <3£    SONS 
EDINBURGH    AND    LONDON 


W.  T.  KEER  &  CO., 
CHICAGO. 


BIOLOGY 
LIBRARY 


PREFACE   TO   FIRST  EDITION 


THE  object  of  this  volume  is  to  put  before  medical  students 
as  succinctly  as  possible  the  essential  facts  of  human 
physiology,  and  to  emphasise  specially  those  parts  of  the 
science  which  are  of  cardinal  importance  in  medicine  and 
surgery. 

Physiology  has  now  become  so  wide  in  scope  that  the 
ordinary  student  must  limit  his  attention  to  matters  which 
have  a  direct  bearing  upon  his  professional  work.  Fortunately 
the  study  of  this  limited  field  affords  ample  opportunity  for 
cultivating  the  scientific  methods  of  observation  and  of 
thought,  which  should  be  gained  by  the  student  before 
approaching  his  clinical  studies. 

In  writing  this  book,  I  have  endeavoured  to  recognise 
and  to  adhere  to  these  limitations,  and  hence  many  parts 
of  physiology  which  occupy  considerable  space  in  the 
ordinary  text-books  have  been  relegated  to  minor  positions, 
while  parts  which  have  a  direct  bearing  upon  the  study 
of  medicine  have  been  purposely  given  a  prominence  which 
their  importance,  when  viewed  from  the  purely  scientific 
standpoint,  would  hardly  warrant. 

Physiology,  like  anatomy,  must  be  studied  practically. 
But  it  is  impossible  for  students  to  perform  more  than  a 
small  number  of  the  experiments  which  are  the  ground- 
work of  the  science,  and  it  is  the  duty  of  the  teacher  to  in- 
dicate such  a  course  of  practical  exercises  as  it  is  possible  for 
them  to  accomplish,  and  which  will  form  a  basis  upon  which 


144J48 


vi  PREFACE  TO  FIRST  EDITION 

to  build  a  really  sound  knowledge.  Such  practical  courses 
are  apt  in  the  student's  mind  to  be  entirely  dissociated  from 
the  systematic  study  of  physiology.  Microscopic  prepara- 
tions are  made  too  often  with  more  consideration  of  the 
methods  employed,  than  of  the  structures  to  be  studied, 
and  experiments  are  performed  as  feats  of  manipulative 
skill,  rather  than  in  relationship  to  what  they  teach. 

I  have  attempted  to  bring  the  practical  and  systematic 
study  into  closer  relationship,  by  constant  references  to  the 
practical  work  which  the  student  must  undertake.  Since 
no  amount  of  reading  will  give  the  real  knowledge  of  the 
histological  methods  which  are  employed,  and  the  structures 
revealed  by  these  methods,  or  of  the  chemical  tests  which 
are  daily  more  and  more  largely  used  by  physicians,  or  of 
the  experimental  procedure  by  which  physiology  has  been 
built  up,  lengthy  descriptions  of  these  methods  seem  un- 
called for ;  and  hence  those  details  which  the  student  must 
master  practically  are  here  merely  touched  upon,  and  he 
is  referred  to  his  practical  exercises. 

Every  student  in  a  recognised  school  now  receives  such 
a  practical  training,  and  the  knowledge  thus  acquired  may 
be  refreshed  by  reference  to  those  books  which  are  used 
as  guides  during  the  prosecution  of  the  practical  work 
— such  books  as  Schafer's  "  Experimental  Physiology," 
Schafer's  "Essentials  of  Histology,"  Halliburton's  "Essen- 
tials of  Chemical  Physiology,"  and  "The  Practical  Physi- 
ology" of  Beddard,  Edkin,  Hill,  M'Leod,  and  Pembrey. 
Descriptions  of  apparatus  have  been  purposely  excluded. 
The  student  must  make  himself  practically  familiar  with 
such  pieces  of  apparatus  as  he  has  to  use  himself,  or  to 
see  used  in  demonstrations.  With  other  apparatus  he  is  not 
concerned.  It  is  as  absurd  that  the  student  of  physiology 
should  burden  his  memory  with  the  construction  of  machines 
he  will  never  see  in  use,  as  that  the  surgeon  should  attempt 


PREFACE  TO   FIRST  EDITION  vii 

to  learn  all  the  various  instruments   which  he   will  never 
require  in  his  practice. 

I  have  pleasure  in  acknowledging  the  kindly  help  which 
I  have  received  from  many  of  my  friends.  Drs.  Gibson, 
Bruce,  Gulland,  Dunlop,  Boyd,  and  Ballantyne  have  given 
me  the  benefit  of  their  criticism  on  those  sections  of  the 
book  which  bear  upon  the  parts  of  medical  science  with 
which  each  of  them  is  specially  conversant.  I  desire  also 
to  record  my  thanks  to  my  class  assistants,  Dr.  Goodall, 
Messrs.  Barcroft,  Kidston,  and  Mackenzie  for  help  in  revising 
the  proofs,  and  to  Mr.  Graham  Brown  for  some  of  the 

histological  diagrams. 

D.  N.  P. 


PREFACE   TO   SECOND   EDITION 


IN  preparing  a  second  edition  of  my  Essentials  of  Physi- 
ology, I  have  endeavoured  to  bring  the  information  up  to 
date,  and  to  increase  the  usefulness  of  the  book  by  rearrang- 
ing and  extending  certain  sections  and  by  the  introduction 
of  many  additional  figures. 

The  purpose  and  character  of  the  book  remain  unchanged. 
It  pretends  to  give  no  more  than  the  Essentials  of  Human 
Physiology,  and  it  is  not  intended  to  replace,  but  only  to 
supplement  the  practical  work  and  demonstrations  from 
which  alone  a  real  knowledge  of  the  subject  can  be  gained. 
To  facilitate  the  linking  of  systematic  and  practical  study, 
references  are  given  to  the  practical  work  which  the  student 
should  do  as  described  in  Schafer's  Class  Work  in  Prac- 
tical Physiology,  and  in  the  author's  Practical  Course  of 
Elementary  Chemical  Physiology. 

My  thanks  are  due  to  Dr.  Goodall  and  to  Mr.  Graham 
Brown,  B.Sc.,  for  the  new  illustrations,  and  to  Dr.  I.  Cameron 
for  reading  the  proof-sheets  and  revising  and  extending  the 
Index. 

D.  N.  P. 


CONTENTS 


PAGE 

INTRODUCTION 1 


PART  I 
SECTION  I 


PROTOPLASM 


SECTION  II 

THE  CELL 14 

SECTION  III 

THE  TISSUES 22 

A.  THE  VEGETATIVE  TISSUES .22 

Epithelium — 

1.  Squamous  Epithelium 22 

2.  Columnar  Epithelium 23 

3.  Glandular  Epithelium 25 

4.  Ciliated  Epithelium 27 

Connective  Tissues — 

1.  Mucoid  Tissue 27 

2.  Fibrous  Tissue 28 

3.  Cartilage 33 

4.  Bone 34 

B.  THE  MASTER  TISSUES— MUSCLE  AND  NERVE          ...  38 

Muscle — 

1.  Physical  and  Chemical  Characters  of  Muscle  at  rest      .  39 

2.  The  Methods  of  making  Muscle  Contract      ...  45 

3.  The  Changes  in  Muscle  during  Contraction  ...  51 

4.  The  Chemical  Changes  in  Muscle  and  the  Source  of  the 

Energy  evolved 67 

5.  Death  of  Muscle 74 

Nerve 75 

The  Neuro-  Muscular  Mechanism 88 

xi 


xii  CONTENTS 

SECTION   IV 

PAGE 

THE  SENSES 95 

A.  COMMON  SENSIBILITY 95 

B.  MUSCLE  AND  JOINT  SENSE 97 

(7.  SPECIAL  SENSES 98 

Tactile  Sense 98 

Temperature  Sense 101 

Vision 102 

Hearing        ..........  129 

Taste 137 

Smell 139 

SECTION  V 

THE  CENTRAL  NERVOUS  SYSTEM— 

Spinal  Nerves 141 

Spinal  Cord 148 

Medulla  and  Cranial  Nerves 154 

Pons  Varolii 160 

Cerebellum 160 

Crura  Cerebri  and  Corpora  Quadrigeinina   ....  167 

Cerebrum     .  168 


PART  II 
THE   NUTRITION    OF   THE   TISSUES 

SECTION   VI 

MANNER  IN  WHICH  NOURISHMENT  is  SENT  TO  THE  TISSUES 
BLOOD  AND  LYMPH 187 

SECTION   VII 

THE  CIRCULATORY  SYSTEM— 

General  Arrangement .  209 

Circulation  through  the  Heart 211 

Circulation  in  the  Blood  and  Lymph  Vessels  .         .         .  244 


CONTENTS  xiii 


SECTION  VIII 

SUPPLY  OF  NOURISHING  MATERIAL  TO  BLOOD  AND  LYMPH,  AND 
ELIMINATION  OF  WASTE  MATTER  FROM  THEM 

PAGE 

KESPIRATION 277 


SECTION  IX 

FOOD  AND  ITS  DIGESTION  AND  UTILISATION 312 

Food 312 

Digestion 324 

Absorption 357 

Heat  Production  and  Temperature  Regulation     .         .         .  360 

Hepatic  Metabolism 366 

General  Metabolism      .         .         .  .         .  .  371 

Dietetics       . 376 

SECTION  X 

INTERNAL  SECRETIONS— Toxic  ACTION  AND  IMMUNITY  .        .  384 

SECTION  XI 

EXCRETION 394 

RENAL  EXCRETION— 

Urine    .         .         .         .         .         .         .         .'"..-.         .  394 

Kidneys        ..........  403 

Renal  Secretion 403 

CUTANEOUS  EXCRETION— 

Sweat  Secretion 409 

Sebaceous  Secretion 410 

Milk  Secretion 410 


PART  III 

SECTION  XII 
REPRODUCTION 413 

APPENDIX 421 

INDEX  429 


REFERENCES 

Practical  Physiology  refers  to  "  Directions  for  Class  Work 
in  Practical  Physiology,  Elementary  Physiology  of 
Muscle  and  Nerve,  and  of  the  Vascular  and  Nervous 
Systems,"  by  E.  A.  SCHAFER,  LL.D.,  F.R.S.,  Pro- 
fessor of  Physiology  in  the  University  of  Edinburgh. 
London:  Longmans,  Green,  &  Co.  1901. 

Chemical  Physiology  refers  to  "  Practical  Course  of  Ele- 
mentary Chemical  Physiology  for  Medical  Students," 
by  D.  NOEL  PATON.  (Second  Edition.)  Edinburgh  : 
William  Green  &  Son.  1905. 


xiv 


Of  THE 

DIVERSITY 
or 


INTRODUCTION 


PHYSIOLOGY  is  really  an  older  science  than  anatomy,  for 
even  before  any  idea  of  pulling  to  pieces,  of  dissecting  the 
animal  machine  had  suggested  itself  to  our  forefathers, 
crude  speculations  in  regard  to  the  causes  and  nature  of  the 
various  vital  phenomena  must  have  been  indulged  in — 
speculations  based  upon  the  vivid  belief  in  the  action  of 
spiritual  agencies,  and  perhaps  unworthy  of  the  name  of 
science.  Still  the  physiology  of  to-day  is  the  offspring  of 
such  speculations. 

Organs  and  Function. — The  first  great  and  true  advance 
was  through  anatomy.  As  that  science  showed  how  the 
body  is  composed  of  distinct  and  different  parts,  it  became 
evident  that  these  parts  or  organs  had  separate  actions  or 
functions ;  and  hence  arose  the  important  conception  of  the 
co-relation  of  organ  and  function. 

From  the  early  metaphysical  speculations  to  such  true 
inductions  was  a  great  stride,  for  a  scientific  method  of 
advance  had  been  established. 

Ever  since  this,  until  quite  recent  times,  physiology  has 
followed  in  the  footsteps  of  anatomy,  or,  to  use  a  more  com- 
prehensive term,  of  morphology.  The  connection  between 
organ  and  function  having  been  demonstrated,  the  ques- 
tions, why  are  these  various  functions  connected  with  the 
respective  organs  ?  why  should  the  liver  secrete  bile,  and  the 
biceps  muscle  contract  ?  next  forced  themselves  upon  the 
attention. 

Tissues  and  Function. — Again  anatomy  paved  the  way 
for  the  explanation.  The  dissecting  knife  and  the  early  and 
defective  microscope  showed  that  the  organs  are  composed 
of  certain  definite  structures  or  tissues,  differing  widely  from 
one  another  in  their  physical  characters  and  appearance,  and, 
as  physiologists  soon  showed,  in  their  functions.  It  now 

1 


2  INTRODUCTION 

became  evident  why  the  liver  secreted  and  the  biceps  con- 
tracted :  the  one  is  composed  of  secreting  tissue,  and  the 
other  of  contracting  tissue. 

Cells  and  Function. — Physiologists  and  anatomists  alike 
devoted  their  energies  to  the  study  of  these  various  tissues, 
and,  as  the  structure  of  the  microscope  improved,  greater  and 
greater  advances  were  made  in  their  analysis,  till  at  length 
Schwann  was  enabled  to  make  his  world-famous  generalisa- 
tion, that  all  the  tissues  are  composed  of  certain  similar 
elements  more  or  less  modified,  which  he  termed  cells,  and 
it  became  manifest  that  the  functions  of  the  different  tissues 
are  due  to  the  activities  of  their  cells. 

The  original  conception  of  the  cell  was  very  different  from 
that  which  we  at  present  hold.  By  early  observers  it  was 
described  as  composed  of  a  central  body  or  nucleus,  sur- 
rounded by  a  granular  cell  substance  with,  outside  all,  a  cell 
membrane.  As  observations  in  the  structure  of  the  cell  were 
extended,  it  soon  became  obvious  that  the  cell  membrane 
was  not  an  essential  part,  and  later,  the  discovery  of  cells 
without  any  distinct  nucleus  rendered  it  clear  that  the 
essential  part  is  the  cell  substance,  and  this  substance  Von 
Mohl  named  protoplasm,  by  which  name  it  is  since  generally 
known. 

Protoplasm  and  Function. — So  far  physiology  had  followed 
in  the  tracks  of  anatomy,  but  now  another  science  became 
her  guide.  Chemistry,  which  during  the  last  century  has 
advanced  with  enormous  strides,  and  has  thrown  such  im- 
portant light  upon  the  nature  of  organic  substances,  now  lent 
her  aid  to  physiology ;  and  morphologists  having  shown  that 
the  vital  unit  is  essentially  simply  a  mass  of  protoplasm,  the 
science  of  life  bids  fair  to  become  the  science  of  the  chemistry 
of  protoplasm. 

The  prosecution  of  physiology  on  these  lines  is  still  in  its 
infancy,  but  already  it  has  changed  the  whole  face  of  the 
science.  Physiology  is  no  longer  the  follower  of  anatomy. 
It  is  become  its  leader,  and  at  the  present  time,  as  we  shall 
afterwards  see,  not  only  the  various  activities,  but  also  the 
various  structural  differences  of  the  different  tissues  are  to  be 
explained  in  terms  of  variations  in  the  chemical  changes  in 
protoplasm. 


INTRODUCTION  3 

In  the  study  of  physiology  this  order  of  evolution  must  be 
reversed,  and  from  the  study  of  protoplasm  the  advance 
must  be  made  along  the  following  lines : — 

1.  Protoplasm — the   physical  basis   of  life;   its  activities 

and  nature. 

2.  Cells. — Manner  in  which  protoplasm  forms   the  vital 

units  of  the  body. 

3.  Tissues. — Manner  in  which  these  are  formed  by  cells. 

Their  structure,  physical  and   chemical  properties, 
and  vital  manifestations. 

4.  Nutrition  of  Tissues. 

a.  Fluids  bathing  the  tissues — 

Blood  and  Lymph. 

b.  Manner  in  which  fluids  are  brought  into  relation- 

ship with  tissues — 
Circulatory  System. 

c.  Manner  in   which   substances    necessary  for   the 

tissues  are  supplied  to  these  fluids — 
Respiratory  System. 
Digestive  System. 
Food,  its  nature  and  quantity. 

d.  Chemical  changes  in  the  tissues  generally — 

Metabolism  and  Heat  Production. 

e.  Manner  in  which  the  waste   products   of  tissues 

are  eliminated — Excretion,  Hepatic,  Renal,  Pul- 
monary, Cutaneous. 

5.  Reproduction  and  Development. 


PART    I 

SECTION    I 

PROTOPLASM 

THE  first  step  in  the  study  of  physiology  must  be  to  acquire 
as  clear  and  definite  a  conception  as  possible  of  the  nature 
of  protoplasmic  activity  in  its  most  simple  and  uncomplicated 
form,  for  in  this  way  an  idea  of  the  essential  and  non-essen- 
tial characteristics  of  life  may  best  be  gained. 

I.  Structure.  —  Protoplasm   is   a   semi-fluid    transparent 
viscous  substance.     It  usually  occurs  in   small  individual 
particles — CELLS — more  or  less  associated,  but  it  may  occur 
as  larger  confluent  masses — PLASMODIA. 

Sometimes  protoplasm  seems  perfectly  homogeneous,  but 
generally  a  reticulated  appearance  can  be  made  out  even  in 
the  living  condition  (Fig.  1),  and  from  this  it  has  been  con- 
cluded that  there  is  a  more  solid  part  arranged  like  the 
fibres  of  a  sponge,  or  like  the  films  of  a  mass  of  soap- 
bubbles,  with  a  more  fluid  interstitial  part.  In  all  proto- 
plasm, therefore,  there  seems  to  be  a  certain  amount  of 
organisation,  and  in  certain  cells  this  organisation  becomes 
very  marked  indeed. 

II.  Physiology. — A   knowledge  of  the  essentials   of   the 
physiology  of  protoplasm  may  be  gained  by  studying  the 
vital  manifestations  of  one  of  the  simplest  of  living  things, 
the  yeast  plant  (Saccharomyces  Cerevisae). 

This  plant  consists  of  very  minute  oval  or  spherical  bodies 
frequently  connected  to  form  chains,  each  composed  of  a 
harder  outer  covering  or  capsule  and  of  a  softer  inner  sub- 
stance which  has  all  the  characters  of  protoplasm. 


PROTOPLASM 


5 


Its  physiology  may  be  studied  by  placing  a  few  torulse  in 
a  solution,  containing  glucose,  C6H1206,  and  urea,  CON2H4, 
with  traces  of  phosphate  of  soda,  Na2HP04,  and  sulphate  of 
potash,  K2SO4. 

If  the  vessel  is  kept  all  night  in  a  warm  place  the  clear 
solution  will  in  the  morning  be  seen  to  be  turbid.  An 
examination  of  a  drop  of  the  fluid  shows  that  the  turbidity 


FIG.  1. — (a)  Foam  structure  of  a  mixture  of  olive  oil  and  cane  sugar ;  (b)  Reticu- 
lated structure  of  Protoplasm  ;  (c)  Reticulated  structure  of  Protoplasm  in  the 
cell  of  an  earth-worm  (after  BUTSCHLI). 

is  due  to  the  presence  of  myriads  of  torulse.  In  a  few  hours 
the  few  torulse  placed  in  the  fluid  have  increased  many 
hundredfold.  The  whole  mass  of  yeast  has  grown  in  amount 
by  the  growth  and  multiplication  of  the  individual  units. 

This  power  of  growth  and  reproduction  under  suitable 
conditions  is  the  essential  characteristic  of  living  matter. 

What  are  the  conditions  necessary  for  the  manifestation  of 
these  phenomena  of  life  ? 


6  HUMAN  PHYSIOLOGY 

1.  If  the  yeast  be  mixed  with  the  solid  constituents  of  the 
solution  in  a  dry  state  no  growth  or  reproduction  occurs. 
Water  is  essential. 

2.  If  the  yeast,  mixed  with  the  solution,  be  kept  at  the 
freezing   point  no  growth   takes   place,   but   this   proceeds 
actively  at  about  36°  C.     A  certain  temperature  is  necessary 
for  the  vitality  of  protoplasm.      In  the   absence  of  these 
conditions,    protoplasm    is    only   potentially   alive,    and    in 
this  state  it  may  remain  for  long  periods  without  under- 
going any  change,  as  in   the  seeds  of  plants  and  in  dried 
bacteria. 

These  conditions  being  present,  in  order  that  the  growth 
of  the  yeast  may  take  place,  there  must  be  : — 

(a)  A  SUPPLY  OF  MATERIAL  from  which  it  can  be  formed. 
(6)  A  SUPPLY  OF  ENERGY  to  bring  about  the  construction. 

The  chemical  elements  in  protoplasm  are  carbon,  hydrogen, 
oxygen,  nitrogen,  sulphur,  and  phosphorus.  These  elements 
are  contained  in  the  ingredients  of  the  solution  used.  If 
yeast  be  sown  in  distilled  water,  even  if  it  be  kept  at  a 
temperature  of  36°  C.,  it  does  not  grow. 

The  energy  is  got  by  the  breaking  down  of  the  sugar, 
C6H1206,  into  alcohol,  C2H6O,  and  carbon  dioxide,  CO2.  Such 
a  breaking  down  of  a  complex  into  simpler  molecules  liberates 
energy,  as  is  well  seen  when  nitro-glycerine  explodes,  breaking 
into  carbon  dioxide,  water,  oxygen,  and  nitrogen — 

2C3H6(N03)3  =  6C02 + 5H20  +  0  +  ON. 

The  energy  can  be  used  for  the  performance  of  work  of 
any  kind,  as,  for  example,  the  work  of  building  up  a  fresh 
quantity  of  the  yeast  plant  out  of  the  substances  contained 
in  the  solution.  The  history  of  the  yeast  plant  shows  that 
protoplasm,  when  placed  in  suitable  conditions,  has  the 
power  of  breaking  down  certain  complex  substances,  and 
of  utilising  the  energy  liberated  for  building  itself  up.  It 
is  this  power  which  has  enabled  living  matter  to  exist  and 
to  extend  over  the  earth. 

How  does  protoplasm  liberate  the  potential  energy  of  such 
substances  ?  At  present  the  answer  to  this  question  cannot 


PROTOPLASM  7 

be  absolutely  definite,  but  in  all  probability  the  yeast  forms 
something  which  acts  upon  glucose,  since,  by  applying 
pressure  to  it,  a  fluid  can  be  extracted  which  contains  a 
substance  having  the  same  action  as  the  living  yeast.  Such 
a  substance  is  called  an  Enzyme  or  Zymin.  These  Zymins 
act  only  in  the  presence  of  water,  and  at  suitable  tempera- 
tures, and  their  action  is  to  accelerate  changes  which  occur 
more  slowly  without  their  presence.  Very  minute  quantities 
can  bring  about  changes  in  large  quantities  of  the  substance 
on  which  they  act,  and  in  acting  they  undergo  no  marked 
change. 

Many  complex  substances  besides  sugar  very  readily  break 
down  and  liberate  energy.  For  instance,  formate  of  lime 
breaks  into  carbonate  of  lime,  carbonic  acid,  and  hydrogen, 
under  different  conditions :  first,  under  the  influence  of  certain 
bacteria,  minute  living  organisms  resembling  yeast  in  many 
particulars;  second,  under  the  influence  of  some  substance 
contained  in  the  bacteria  after  they  are  killed;  and  lastly, 
in  the  presence  of  finely  divided  iridium,  rhodium,  and 
ruthenium — i.e.  the  same  change  is  brought  about  by  a 
living  organism,  by  a  substance  contained  in  it,  and  by  a 
metal. 

Hence  mere  vibrations  of  molecules,  occurring  in  different 
ways  and  in  different  substances,  may  be  sufficient  to  bring 
about  these  changes,  and  however  the  change  is  brought 
about  the  result  is  to  set  free  energy.  Such  a  process  has 
been  termed  Catalysis. 

Living  yeast  differs  from  these  dead  substances  simply  in 
the  fact  that  it  uses  the  energy  liberated  from  the  glucose. 
In  virtue  of  this,  the  yeast  has  the  power  of  repair  and  of 
growth. 

But  protoplasm  is  also  constantly  breaking  down,  and  if 

yeast  be  kept  at  a  suitable  temperature  in  water  without  any 
supply  of  material  for  construction,  it  gives  off  carbon  dioxide 
and  decreases  in  bulk  on  account  of  these  disintegrative 
changes.  These  are  as  essential  a  part  of  the  life  of  living 
matter  as  the  building-up  changes,  and  it  is  only  when  they 
are  in  progress  that  the  latter  are  possible. 

Protoplasm  (living  matter)  is  living  only  in  virtue  of  its 


8  HUMAN  PHYSIOLOGY 

constant  chemical  changes,  metabolism,  and  these  changes 
are  on  the  one  hand  destructive  (katabolic),  on  the  other 
constructive  (anabolic).  Living  matter  thus  differs  from 
dead  matter  simply  in  this  respect,  that  side  by  side  with 
destructive  changes,  constructive  changes  are  always  going 
on,  whereby  its  amount  is  maintained  or  increased. 

Hence  our  conception  of  living  matter  is  not  of  a  definite 
chemical  substance,  but  of  a  substance  constantly  undergoing 
internal  changes.  It  might  be  compared  to  a  whirlpool  con- 
stantly dragging  things  into  its  vortex,  and  constantly  throw- 
ing them  out  more  or  less  changed,  but  itself  continuing 
apparently  unchanged  throughout.  Hoppe-Seyler  expresses 
this  by  saying :  "  The  life  of  all  organisms  depends  upon,  or, 
one  can  almost  say,  is  identical  with  a  chain  of  chemical 
changes."  Foster  puts  the  same  idea  in  more  fanciful 
language :  "  We  may  speak  of  protoplasm  as  a  complex  sub- 
stance, but  we  must  strive  to  realise  that  what  we  mean  by 
that  is  a  complex  whirl,  an  intricate  dance,  of  which,  what 
we  call  chemical  composition,  histological  structure,  and  gross 
configuration  are,  so  to  speak,  the  figures." 

The  rate  of  these  changes  may  be  quickened  or  slowed  by 
changes  in  the  surroundings,  and  such  changes  are  called 
stimuli.  If  the  stimulus  increases  the  rate  of  change,  it  is 
said  to  excite ;  if  it  diminishes  the  rate  of  change,  it  is  said 
to  depress.  Thus  the  activity  of  the  changes  in  yeast  may 
be  accelerated  by  a  slight  increase  of  the  temperature  of  the 
surrounding  medium,  or  it  may  be  depressed  by  the  addition 
of  such  a  substance  as  chloroform  water. 

While  the  continuance  of  these  chemical  changes  in  proto- 
plasm is  life,  their  stoppage  is  death.  For  the  continuance 
of  life  the  building-up  changes  must  be  in  excess  of  or  equal 
to  the  breaking-down,  and  when  failure  in  the  supply  or  in 
the  utilisation  of  the  material  used  in  construction  occurs, 
the  protoplasm  dwindles  and  disintegrates.  Death  is  sudden 
when  the  chemical  changes  are  abruptly  stopped,  slow  when 
the  anabolic  changes  are  interfered  with.  The  series  of 
changes  which  occur  between  the  infliction  of  an  incurable 
injury  and  complete  disintegration  of  the  tissue  constitute 
the  processes  of  Necrobiosis,  and  their  study  is  of  importance 
in  pathology. 


PROTOPLASM  9 

III.  Chemistry. — It  is  impossible  to  analyse  such  an  ever- 
changing  substance  as  protoplasm,  and  although  what  is  left 
when  these  chemical  changes  are  stopped  can  be  examined, 
such  analyses  give  little  insight  into  the  essential  nature  of 
the  living  matter. 

That  substances  of  great  complexity  take  part  in  the  con- 
stant whirl  is  shown  by  the  analyses  of  what  is  left  after 
death.  Five  or  six  elements — carbon,  hydrogen,  oxygen, 
nitrogen,  sulphur,  and  phosphorus  are  present,  and  these 
are  linked  together  to  form  molecules  of  enormous  size. 

Water  is  the  most  abundant  constituent  of  protoplasm, 
amounting,  as  it  does,  to  about  75  per  cent. 

The  Solids,  constituting  the  remaining  25  per  cent.,  con- 
sist chiefly  of  a  series  of  bodies  closely  allied  to  one  another 
and  called  "  chief  substances  "  or  Proteids.  In  addition  to 
these,  certain  inorganic  salts  are  found  in  the  ash  when 
protoplasm  is  burned,  indicating  the  presence  of  POTASSIUM 
and  CALCIUM  along  with  PHOSPHORUS  and  SULPHUR.  Small 
and  varying  quantities  of  FATTY  SUBSTANCES,  and  of  CARBO- 
HYDRATES, with  traces  of  a  number  of  other  organic  sub- 
stances which  need  not  here  be  enumerated,  are  also  usually 
present. 

Of  these  substances  the  Proteids  alone  have  to  be  con- 
sidered here,  since  they  constitute  the  really  important  part 
of  the  material. 

PROTEIDS 

White  of  egg  may  be  taken  as  an  example  of  such  pro- 
teids  dissolved  in  water  with  some  salts.  If  the  salts  be 
separated,  and  the  water  carefully  driven  off  at  a  low 
temperature,  a  pure  proteid  is  left. 

(A)  Physical  Characters. — Proteids  have  a  white,  yellow, 
or  brownish  colour.  In  structure  they  are  usually  amorphous, 
but  many  have  been  prepared  in  a  crystalline  condition, 
and  it  is  probable  that  all  may  take  a  crystalline  form. 
The  crystals  vary  in  shape,  being  usually  small  and  needle- 
like,  but  sometimes  forming  larger  rhombic  plates.  Some 
proteids  are  soluble  in  water,  others  require  the  presence 
of  neutral  inorganic  salts,  others  of  an  acid  or  alkali, 
while  some  are  completely  insoluble  without  a  change 


io  HUMAN   PHYSIOLOGY 

in  their  constitution.  All  are  insoluble  in  alcohol  and 
ether. 

When  in  solution,  or  apparent  solution,  many  of  the  pro- 
teids  do  not  dialyse  through  an  animal  membrane,  and  they 
are  hence  called  Colloids.  Other  colloidal  bodies  reacting 
much  like  the  proteids  have  been  prepared  synthetically  by 
chemists — e.g.  by  heating  together  amido-benzoic  acid  and 
phosphoric  anhydride.  Like  other  colloids  they  tend  to 
coagulate,  forming  a  clot  just  as,  for  instance,  silicic  acid 
may  clot  when  carbon  dioxide  is  passed  through  its  solution. 

All  proteids  rotate  the  plane  of  polarised  light  to  the  left. 

(B)  Chemistry. — Proteids  contain  the  following  chemical 
elements :  carbon,  hydrogen,  oxygen,  nitrogen,  and  sulphur, 
in  about  the  following  percentage  amounts : — 

c.  H.  N.  s.  o. 

52  7  16  1  24 

It  is  important  to  remember  the  amounts  of  nitrogen  and 
carbon,  since  proteids  are  the  sole  source  of  the  former 
element  in  the  food  and  an  important  source  of  the  latter. 

As  regards  the  number  of  atoms  of  these  elements  which 
go  to  form  a  single  molecule,  information  has  been  obtained 
by  studying  compounds  with  various  metals.  The  following 
probable  formula  of  the  molecule  of  the  chief  proteid  of  the 
white  of  egg  is  given  simply  to  show  how  complex  these 
substances  are :  C^Hg^N^OgA. 

Our  knowledge  of  the  constitution  of  the  molecule  is  still 
very  imperfect. 

The  simplest  bodies  having  the  characters  of  proteids  are 
the  Protamines,  basic  substances  which  are  found  in  the  heads 
of  spermatozoa,  combined  with  nucleic  acid  (p.  12).  When 
they  are  broken  down  they  yield  chiefly  Hexone  Bases,  so 
called  from  having  six  atoms  of  carbon  in  their  molecule. 
These  are — 

DIAMIDO  ACIDS — acids  having  two   ainidogens  —  NH2  — 
in  their  molecule.     The  most  important  is : — 

(1)  Arginin,  diamido-valerianic  acid  linked  to  guani- 

din,  and  therefore  allied  to  creatin.    (See  p.  43.) 

(2)  Lysin,  diamido-caproic  acid,  and 

(3)  Histidin,  a  substance  of  unknown  constitution. 


PROTOPLASM  1 1 

The  ordinary  proteids  are  built  on  the  same  plan  as  the 
protamines,  but  have  complex  side  chains  of  MONAMIDO 
ACIDS  linked  to  the  hexone  bases.1 

Of  these  monamido  acids  perhaps  the  best  known  are — 

(1)  Leucin  (amido-caproic  acid)— C5H10NH9CO.OH. 

(2)  Tyrosin,  in  which  amido-proprionic  acid  is  linked 

to  an  aromatic  nucleus.2 


H  NH20 


c«  H4  ]  _c_ C— C— 0— H 


H    H 


CLASSIFICATION   OF   THE   PROTEIDS 

(A)  Simple  Proteids. — 1.  Native  Proteids. — These  pro- 
teids, either  alone,  or  combined  with  certain  other  sub- 
stances, are  constant  ingredients  of  dead  protoplasm,  and 
of  the  fluid  constituents  of  the  body.  They  are  distinguished 
from  all  other  proteids  by  being  coagulated  on  heating. 

There  are  two  groups  —  Globulins  and  Albumins  —  the 
former  characterised  by  being  insoluble  in  distilled  water, 
by  requiring  the  presence  of  a  small  quantity  of  a  neutral 
salt  to  form  a  solution,  and  by  being  precipitated  from 
solution  by  half  saturating  with  sulphate  of  ammonia. 

2.  Proteoses  (Proteids  with  a  less  complex  molecule  than 
albumins  and  globulins). — They  may  be  formed  from  albu- 
mins (albumoses)  and  globulins  (globuloses),  by  the  action 
of  superheated  steam  and  during  digestion.  Under  the 
influence  of  these  agents,  the  complex  molecule  splits  into 
simpler  molecules  and  takes  up  water. 

These  proteoses  form  a  series  between  the  original  proteids 
on  the  one  hand,  and  the  peptones  or  simplest  proteids  on 
the  other.  They  may  be  divided  into  two  classes  : — 

(a)  Those  nearly  allied  to  the  original  proteids — Proto- 

1  For  the  tests  for  proteids  and  the  methods  of  distinguishing  the  individual 
proteids,  see  Chemical  Physiology,  p.  3. 

2  For  some  elementary  facts  of  organic  chemistry  necessary  for  the  com- 
prehension of  these  details,  see  Appendix,  p.  421  et  seq. 


12  HUMAN  PHYSIOLOGY 

proteoses,  which  are  precipitated  in  a  saturated  solution  of 
common  salt,  NaCl. 

(6)  Those  more  nearly  allied  to  the  peptones  —  Deutero- 
proteoses,  which  are  not  precipitated  in  a  saturated  solution 
of  NaCl,  but  are  precipitated  by  a  saturated  solution  of 
sulphate  of  ammonia. 

3.  Peptones.  —  These  are  the  ultimate  products  of  the 
action  of  gastric  juice  on  proteids.  Their  characteristic 
reaction  is  their  solubility  in  hot  saturated  sulphate  of 
ammonium  solution.  They  diffuse  very  readily  through 
an  animal  membrane. 

(B)  Conjugated  Proteids.— Proteids  have  a  great  tendency 
to  link  with  other  substances — 

(1)  Proteates  are  formed  by  linking  acids  or  alkalies  to  the 
native  proteids. 

(2)  Nucleins,  so  called  because   their   existence  was  first 
demonstrated  in  the  nuclei  or  central  parts  of  the  cells  of 
the   body,   may   readily   be    split  into   a  proteid   part  and 
into    nucleic    acid,   a    phosphorus  -  containing    material   of 
definite  composition,  having  an  acid  reaction,  and  containing 
about  10  per  cent,   of  phosphorus.      In  certain  places  the 
amount  of  nucleic  acid  is  large  in  proportion  to  the  proteid, 
in  others  it  is  small.     The  term  nuclein  is  usually  confined 
to  the  former,  nucleo-albumin  to  the  latter  of  these.     From 
the  pure  nucleic  acid,  which  occurs  along  with  protamine 
in  the  heads  of  spermatozoa,  to  the  proteids  almost  free  of 
phosphorus  there  is  a  continuous  series. 

Nucleic  acid  when  decomposed  yields  phosphoric  acid  and 
a  series  of  bodies  called  the  Purin  bodies  which  belong  to  the 
class  of  diureides,  and  consist  of  two  more  or  less  modified 
urea  molecules  linked  together  by  the  radicle  usually  of 
acrylic  acid  (see  p.  397). 

(3)  Pseudo-Nucleins. — Other  compounds  of  proteids  with 
phosphorus-containing  molecules  occur  which  do  not  yield 
purin  bodies  when  decomposed.     Of  these  vitellin,  the  pro- 
teid of  the  yolk  of  egg,  is  an  example. 

(4)  Histones  are  proteids  linked  to  protamine.    They  occur 
in  the  globin  which  may  be  separated  from  blood  pigment. 
They  have  a  basic  reaction. 

(5)  Glyco-proteids. — Proteids  are  linked  with   sugar-like 


PROTOPLASM  13 

substances  to  form  compounds.     The  best-known  example  is 
Mucin  (see  p.  26). 

(6)  Ferro-proteids. — In  the  pigment  of  the  blood  (Haemo- 
globin) proteids  occur  linked  to  an  iron-containing  molecule 
(p.  199). 

(7)  In  horn  (Keratin)  a  sulphur- containing  molecule  joined 
to  the  proteid  gives  the  special  characters  to  the  substance 
(p.  23). 


SECTION  II 
THE  CELL 

PROTOPLASM  occurs  in  the  animal  body  as  small  separate 
masses  or  CELLS.  These  vary  considerably  in  size,  but,  on 
an  average,  they  are  from  7  to  20  micro-millimetres  in 
diameter.  The  advantage  of  this  subdivision  is  obvious. 
It  allows  nutrient  matter  to  reach  every  particle  of  the  proto- 
plasm. In  all  higher  animals  each  CELL  has  a  perfectly 
definite  structure.  It  consists  of  a  mass  of  protoplasm,  in 
which  is  situated  a  more  or  less  defined  body,  the  nucleus. 

(A)  Cell  Protoplasm.— This  has  the  structure  .already 
described  under  protoplasm,  and  in  different  cells  the  reti- 
culum  or  cytomitoma  is  differently  arranged.  In  some  cells 
there  is  a  condensation  of  the  reticulum,  round  the  periphery, 
to  form  a  sort  of  cell  membrane. 

At  some  point,  in  the  protoplasm  of  many  cells,  one  or  two 
small  spherical  bodies,  the  centrosomes  (Fig.  2),  are  found, 
from  which  rays  pass  out  in  different  directions.  For  the 
detection  of  these  bodies  special  methods  of  staining  and  the 
use  of  very  high  magnifying  powers  are  required.  They  will 
be  again  considered  when  dealing  with  the  reproduction  of 
cells. 

The  cell  protoplasm  frequently  contains  granules,  either 
formed  in  the  protoplasm  (p.  25),  or  consisting  of  material 
ingested  by  the  cell. 

In  the  protoplasm,  vacuoles  are  sometimes  found,  and 
from  a  study  of  these  vacuoles  in  protozoa,  it  appears  that 
they  are  often  formed  round  material  which  has  been  taken 
into  the  protoplasm,  and  that  they  are  filled  with  a  fluid 
which  can  digest  the  nutritious  part  of  the  ingested  par- 
ticles. In  some  cells  vacuoles  may  appear  in  the  process 
of  disintegration. 

14 


THE   CELL  15 

In  certain  cells  protoplasm  undergoes  changes  in  shape. 
This  may  well  be  studied  in  the  white  cells  in  the  blood  of 
the  frog  or  newt.  Processes  are  pushed  out,  and  these  are 
again  withdrawn,  or  the  whole  cell  may  gradually  follow  the 
process,  and  thus  change  its  position.  The  processes  are 
called  pseudopodia  (false  feet),  and  the  mode  of  movement, 


FIG.  2. — (a)  White  Cell  from  blood  to  show  the  centrosome  and  nucleus ;  (6)  Egg 
Cell,  dividing,  shows  reticulated  structure  of  Protoplasm,  two  centrosomes 
and  nuclear  fibres  in  mitosis  (division  of  nucleus). 

from  its  resemblance  to  that  seen  in  the  amoeba,  is  called 
amoeboid. 

The  part  played  by  reticulum  and  hyaloplasm  in  these 
movements  is  not  clearly  understood.  The  pseudopodia  are 
at  first  free  of  reticulum ;  but  \vhether  the  hyaloplasm  is 
pressed  out  by  contraction  of  the  reticulum,  or  whether  it 
actively  flows  out,  is  not  known.  In  some  cells  among  the 


1 6  HUMAN  PHYSIOLOGY 

protozoa  movements  take  place  along  some  definite  line, 
and  the  reticulum  is  arranged  more  or  less  parallel  to  the 
line  of  movement.  Such  contractile  processes,  from  their 
resemblance  to  muscles,  have  been  termed  myoids.  In 
other  protozoa  the  pseudopodia  may  manifest  a  to-and-fro 
rhythmic  waving  movement,  which  may  cause  the  cell  to 
be  moved  along,  or  may  cause  the  adjacent  fluid  to  move 
over  the  cell.  Such  mobile  processes  when  permanent  have 
been  called  cilia. 

These  movements  are  modified  by  the  various  STIMULI 
which  modify  the  activity  of  the  chemical  changes  in  the 
protoplasm  (p.  8).  Thus  cooling  diminishes,  and  finally 
stops  them.  Gentle  heat  increases  them,  but  when  a  certain 
temperature  is  reached,  they  are  stopped.  Drying  and 
various  drugs,  such  as  chloroform,  quinine,  &c.,  also  arrest 
the  movements. 

Changes  in  the  surroundings  may  cause  either  contraction 
or  expansion,  may  repel  or  attract.  When  a  repelling  or 
attracting  influence,  a  positive  or  negative  stimulus,  acts  at 
one  side  of  the  cell — unilateral  stimulation — it  may  lead  to 
movement  of  the  cell  away  from  it  or  towards  it.  Movements 
are  produced  by  various  chemical  substances  (chemiotaxis), 
or  by  light  (photo taxis)  or  by  electricity  (galvanotaxis).  If 
the  action  is  towards  the  stimulus,  it  is  said  to  be  positive, 
if  away  from  it  negative. 

Chemiotaxis  is  the  attraction  or  repulsion  produced  by  one- 
sided application  of  chemical  stimuli.  This  is  well  seen  in 
the  plasmodial  masses  of  aethalium  septicum  which  grow  on 
tan.  Oxygen  and  water  both  attract  it  towards  them,  and 
exercise  a  positive  chemiotaxis.  It  is  also  seen  in  the  stream- 
ing of  the  white  cells  of  the  blood  to  disintegrating  tissues, 
or  to  various  micro-organisms  which  have  to  be  destroyed  to 
prevent  their  poisoning  the  organism,  and  in  the  attraction 
exercised  by  the  ovum  upon  the  male  element  in  repro- 
duction. 

Barotaxis  is  the  effect  of  unilateral  pressure  or  mechanical 
stimulation.  Many  protozoa  appear  quite  unable  to  leave 
the  solid  substance — e.g.  the  microscope  slide — with  which 
they  are  in  contact,  the  unilateral  pressure  seeming  to  cause 
a  positive  attraction  in  that  direction. 


THE   CELL  17 

Phototaxis. — Light,  which  plays  so  important  a  part  in 
directing  the  movements  of  the  higher  plants,  also  acts 
positively  or  negatively  on  many  unicellular  organisms. 
Thus  the  swarm  spores  of  certain  algse  are  positively  at- 
tracted by  moderate  illumination,  streaming  to  the  source 
of  light,  while  they  are  negatively  stimulated  by  strong 
light,  and  stream  away  from  it.  Light  also  plays  an  im- 
portant part  in  directing  the  movements  of  certain  bacteria. 

Thermotaxis. — The  unilateral  influence  of  temperature  is 
well  seen  in  the  plasmodium  of  sethaliuin  septicum  which 
streams  from  cold  water  towards  water  at  a  temperature  of 
about  30°  C. 

Gfalvanotaxis. — As  would  naturally  be  expected  from  its 
stimulating  action,  a  current  of  electricity  has  a  most  power- 
ful effect  in  directing  the  movements  of  many  cells.  Certain 
infusoria  when  brought  between  the  poles  of  a  galvanic 
battery  may  be  observed  to  stream  towards  the  negative  pole. 

The  effects  of  this  unilateral  stimulation  are  of  great 
importance  in  physiology  and  pathology,  since  they  explain 
the  streaming  of  leucocytes  to  attack  micro-organisms  and 
other  poisons  to  the  animal  body,  and  since  they  seem  to 
explain  many  of  the  apparently  volitional  acts  of  unicellular 
organisms.  Many  of  these  organisms  appear  to  definitely 
select  certain  foods,  but  in  reality  they  are  simply  compelled 
towards  them  by  this  unilateral  stimulation. 

(B)  Nucleus. 

(1)  Structure. — The  nucleus,  seen  with  a  moderate  mag- 
nifying power,  appears  in  most  cells  as  a  well-defined  circular 
or  oval  body  situated  towards  the  centre  of  the  cell.  (Figs.  1 
and  2.)  Sometimes  it  is  obscured  by  the  surrounding  proto- 
plasm. It  has  a  granular  appearance,  and  usually  one  or 
more  clear  refractile  bodies — the  nucleoli — are  seen  within 
it.  It  stains  deeply  with  many  reagents  of  a  basic  reaction, 
such  as  hsematoxylin,  carmine,  methylene  blue,  &c.  In 
some  cells  the  nucleus  is  irregular  in  shape  (Fig.  2),  and  in 
some  it  is  broken  up  into  a  number  of  pieces,  giving  the  cell 
a  multi-nucleated  character. 

It  is  usually  composed  of  a  mass  of  fibres  arranged  in  a 

2 


1 8  HUMAN   PHYSIOLOGY 

complicated  network  (Fig.  1),  and  it  is  these  fibres  which 
have  a  special  affinity  for  basic  stains.  Between  these  fibres 
is  a  more  fluid  material  which  may  be  called  the  nuclear 
plasma.  Digestion  in  the  stomach  removes  the  nuclear 
plasma,  but  leaves  the  network  unacted  upon. 

The  nuclear  fibres  are  made  up  of  two  substances,  a  ground 
substance  or  matrix,  which  has  no  affinity  for  colouring  matter, 
and  which  is  hence  called  achromatin  substance ;  and  a  sub- 
stance with  a  marked  affinity  for  various  dyes,  set  in  the 
former  in  a  series  of  particles  variously  arranged,  and  dis- 
tinguished as  chromatin.  It  is  the  staining  of  this  material 
which  colours  the  nucleus  so  deeply. 

The  chromatin  substance  contains  a  large  amount  of 
nucleic  acid,  and  its  richness  in  phosphorus  has  been  demon- 
strated by  treating  the  cells  with  ammonium  molybdate  and 
pyrogallol,  which  colours  parts  rich  in  phosphorus  of  a  brown 
or  black  tint. 

The  nuclear  fibres  vary  in  their  arrangement  in  different 
cells.  Usually  they  form  a  network,  but  occasionally  they 
are  disposed  as  a  continuous  skein.  In  nuclei,  with  the 
former  arrangement  of  fibres,  swellings  may  be  observed 
where  the  fibres  unite  with  one  another — the  nodal  swellings. 
The  nucleoli  are  probably  not  all  of  the  same  nature,  and 
some  may  be  simply  specially  well-marked  nodal  swellings. 
The  resting  nucleus  appears  to  be  surrounded  by  a  distinct 
nuclear  membrane,  which  is,  however,  probably  really  a 
basket-like  interlacement  of  the  fibres  at  the  periphery. 

(2)  Functions. — The  part  taken  by  the  nucleus  in  the 
general  life  of  the  cell  is  not  yet  fully  understood.  1st,  It 
exercises  an  influence  on  the  nutritive  processes,  since  it 
has  been  observed  in  certain  of  the  large  cells  in  lower 
organisms  that  a  piece  of  the  protoplasm  detached  from  the 
nucleus  ceases  to  grow,  and,  after  a  time,  dies.  Important 
interchanges  of  material  go  on  between  the  nucleus  and  the 
protoplasm.  2nd,  It  is  the  great  reproductive  organ  of  the 
cell,  probably  playing  an  important  part  in  transmitting  in- 
herited characters. 

Reproduction  of  Cells. — Cells  do  not  go  on  growing  in- 
definitely. When  they  reach  a  certain  size  they  generally 


THE  CELL 


either  divide,  to  form  two  new  cells,  or  they  die  and  undergo 
degenerative  changes.  The  reason  of  this  is  possibly  to  be 
found  in  the  well-known  physical  fact  that,  as  a  sphere 
increases  in  size,  the  mass  increases  more  rapidly  than  the 
periphery.  Hence,  as  a  cell  becomes  larger  and  larger,  the 
surface  for  nourishment  becomes  smaller  and  smaller  in 
relationship  to  the  mass  of  material  to  be  nourished.  Pro- 
bably the  altered  metabolism  so  produced  sets  up  the 
changes  which  lead  to  the  division  of  the  cell.  These 
changes  have  now  been  very  carefully  studied  in  a  large 
number  of  cells,  and  it  has  been  shown  that  the  nucleus 
generally  takes  a  most  important  part  in  division. 


(3) 

FIG.  3. — Nucleus  in  Mitosis  :  (1)  Convoluted  stage  ;  (2)  Monaster  stage ; 
(3)  Dyaster  stage ;  (4)  Complete  division. 

Mitosis. — In  a  cell  about  to  divide,  the  first  change  is  a 
general  enlargement  of  the  nucleus.  At  the  same  time  the 
centrosome  becomes  double,  and  the  two  portions  travel  from 
one  another,  but  remain  united  by  delicate  lines  to  form  a 
spindle-shaped  structure  (Fig.  3,  1).  These  lines  may  be 


20  HUMAN   PHYSIOLOGY 

actual  fibres  or  they  may  be  lines  of  movement  in  the  proto- 
plasm. The  spindle  passes  into  the  centre  of  the  nucleus,  and 
seems  to  direct  the  changes  in  the  reticulum.  The  nuclear 
membrane  disappears,  and  the  nucleus  is  thus  not  so  sharply 
marked  off  from  the  cell  protoplasm.  The  nucleoli  and 
nodal  points  also  disappear,  and  with  them  all  the  finer 
fibrils  of  the  network,  leaving  only  the  stouter  fibres,  which 
are  now  arranged  either  in  a  skein  or  as  loops  with  their 
closed  extremity  to  one  pole  of  the  nucleus  and  their  open 
extremity  to  the  other.  The  nucleus  no  longer  seems  to 
contain  a  network,  but  appears  to  be  filled  with  a  con- 
voluted mass  of  coarse  fibres,  and  hence  this  stage  of  nuclear 
division  is  called  the  convoluted  stage. 

The  spindle  continues  to  grow  until  it  occupies  the  whole 
length  of  the  nucleus.  The  two  centrosomes  are  now  very 
distinct,  and  from  them  a  series  of  radiating  striae  extends 
out  into  the  protoplasm  of  the  cell. 

The  nuclear  loops  of  fibres  break  up  into  short,  thick 
pieces;  and  these  become  arranged  around  the  equator  of 
the  spindle  in  a  radiating  manner,  so  that  when  the  nucleus 
is  viewed  from  one  end  it  has  the  appearance  of  a  rosette  or 
a  conventional  star.  This  stage  of  the  process  is  hence  often 
called  the  single  star  or  monaster  stage  (Fig.  3,  2). 

Each  loop  now  splits  longitudinally  into  two,  the  divisions 
lying  side  by  side  (Fig.  3,  2). 

The  next  change  consists  in  the  separation  from  one 
another  of  the  two  halves  of  the  split  loops — one  half  of  each 
passing  up  towards  the  one  polar  body,  the  other  half  passing 
towards  the  other.  It  is  the  looped  parts  which  first  separate 
and  which  lead  the  way — the  open  ends  of  the  loops  remain- 
ing in  contact  for  a  longer  period,  but,  finally,  also  separating. 
In  this  way,  around  each  polar  body,  a  series  of  looped  fibres 
gets  arranged  in  a  radiating  manner,  so  that  the  nucleus 
now  contains  two  rosettes  or  stars,  and  this  stage  of  division 
is  hence  called  the  dy aster  stage  (Fig.  3,  3). 

The  single  nucleus  is  now  practically  double.  Gradually 
in  each  half  finer  fibres  develop  and  produce  the  reticular 
appearance.  Nuclear  nodes,  nucleoli,  and  the  nuclear  mem- 
brane appear,  and  thus  two  resting  nuclei  are  formed  from  a 
single  nucleus.  Between  these  two  nuclei  a  delicate  line 


THE  CELL  21 

appears,  dividing  the  cell  in  two,  and  the  division  is  accom- 
plished (Fig.  3,  4). 

The  network  of  the  nucleus  of  actively  dividing  cells  is 
rich  in  nucleic  acid,  but  in  cells  which  have  ceased  to  divide, 
in  which  the  nucleus  has  ceased  to  exercise  its  great  repro- 
ductive function,  the  amount  of  phosphorus — i.e.  of  nucleic 
acid — diminishes,  and  may  be  actually  less  than  the  amount 
in  the  cell  protoplasm. 

Amitotic  Division. — In  some  cells  the  nucleus  does  not 
appear  to  take  an  active  part,  the  cell  dividing  without  the 
characteristic  changes  above  discussed. 


SECTION    III 

THE  TISSUES 

FROM  the  protoplasm  of  the  cells  the  various  tissues  of  the 
body — bone,  cartilage,  muscle,  £c.,  are  formed.  The  structure 
of  these  must  be  studied  practically;  all  that  will  be  at- 
tempted here  is  to  indicate  how  they  are  formed  from  the 
primitive  cell. 

The  human  body  is  originally  a  single  cell,  and  from  this, 
by  division,  a  mass  of  simple  cells  is  produced.  In  the 
embryo,  these  cells  get  arranged  in  three  layers — an  outer, 
a  middle,  and  an  inner — the  epiblast,  mesoblast,  and  hypo- 
blast. 

(A)  THE  YEGETATIYE  TISSUES 

The  Vegetative  Tissues  are  those  which  support,  bind  to- 
gether, protect,  and  nourish  the  body.  They  may  be  divided 
into  the  Epithelial  Tissues,  formed  from  the  epiblast  or  hypo- 
blast,  and  consisting  of  cells  placed  upon  surfaces,  and  the 
Connective  Tissues  developed  from  the  mesoblast,  and  con- 
sisting chiefly  of  formed  material  between  cells. 

I.  EPITHELIUM 

1.  Squamous  Epithelium— 

(a)  Simple  Squamous  Epithelium. — This  is  seen  lining  the 
air  vesicles  of  the  lungs.  It  consists  of  a  single  layer  of  flat, 
scale-like  cells,  each  with  a  central  nucleus.  The  outlines 
of  these  cells  are  made  manifest  by  staining  with  nitrate 
of  silver,  which  blackens  the  cement  substance  between  the 
cells. 

(6)  Stratified  Squamous  Epithelium  (Fig.  4). — The  skin 
and  the  lining  membrane  of  the  mouth  and  gullet  are  covered 
by  several  layers  of  cells.  The  deeper  cells  divide,  and  as 
the  young  ones  get  pushed  upwards  towards  the  surface,  and 
away  from  the  nourishing  fluids  of  the  body,  the  chemical 


THE   TISSUES  23 

changes  are  interfered  with,  and  the  protoplasm  undergoes 
a  change  into  a  body  closely  allied  in  composition  to  the 
proteids,  keratin.  This  substance  is  a  hard,  horny  material, 
which  is  well  seen  in  the  nails  and  hair,  and  in  the  horns  and 
hoofs  of  certain  animals.  It  first  makes  its  appearance  as  a 
number  of  little  masses  or  granules  in  the  cells,  and  these 
run  together  to  fill  the  cells,  which  from  pressure  become 
flattened  out  into  thin  scales. 


FIG.  4. — Stratified  Squamous  Epithelium  from  the  cornea. 

Keratin  forms  an  admirable  protective  covering  to  the 
body,  not  only  on  account  of  its  hardness  and  toughness,  but 
because  poisons  cannot  readily  pass  through  it,  and  also 
because  it  is  not  easily  acted  on  by  chemicals.  Like  the 
proteids,  it  contains  carbon,  hydrogen,  oxygen,  nitrogen,  and 
sulphur;  and  the  first  four  of  these  elements  are  in  about 
the  same  proportion  as  in  the  proteids.  But  the  sulphur  is 
in  greater  proportions  (3  to  5  per  cent.),  and  readily  enters 
into  combination  with  various  substances.  Hence,  lead  solu- 
tions, which  give  the  black  sulphide  of  lead,  colour  keratin 
black,  and  are  largely  used  to  dye  the  hair  (see  Chemical 
Physiology,  p.  6).  A  slightly  modified  stratified  squamous 
epithelium  lines  the  urinary  passages. 

2.  Columnar  Epithelium  (Fig.  5,  a). — The  inmost  set  of 
cells  in  the  embryo,  lining  the  stomach  and  intestine,  elongate 


24  HUMAN  PHYSIOLOGY 

at  right  angles  to  their  plane  of  attachment,  and  become 
columnar  in  shape.  The  free  border  of  the  cells  looks  like 
a  hem,  an  appearance  which  is  due  to  a  series  of  short  rods 


Fio.  5.— (a)  Columnar  Epithelium  from  the  small  intestine;  (b)  Ciliated 
Epithelium  from  the  trachea. 

placed  side  by  side.  Probably  this  is  a  special  development 
of  the  reticulum  of  the  protoplasm.  The  great  function  of 
this  form  of  epithelium  is  to  take  up  the  digested  matter 
from  the  stomach  and  intestine,  and  to  pass  it  on  to  the 
blood. 

Among  these  columnar  cells  a  certain  number  of  peculiarly 


Fio.  6. — A  Zymin-secreting  Gland,  to  show  duct  and  acinus  lined  with  secreting 
cells  containing  zymogen  granules. 

modified  cells,  chalice  cells,  are  always  found.  They  are 
larger  than  the  columnar  cells,  and  somewhat  pear-shaped, 
being  attached  by  their  small  extremity.  Their  protoplasm 


THE   TISSUES  25 

is  collected  at  their  point  of  attachment,  while  the  body  of 
the  cell  is  filled  with  mucin,  a  clear,  transparent  material. 

3.  Glandular  Epithelium. — A  number  of  cells,  having  for 
their  function  the  production  of  some  material  which  is  to  be 
excreted  from  the  cell,  are  arranged  as  the  lining  of  depres- 
sions, the  glands. 

The  simplest  form  of  gland  is  the  simple  tubular — a  test- 
tube-like  depression,  lined  by  secreting  cells.  Instead  of 
being  simple,  the  tube  may  be  branched,  when  the  gland  is 
said  to  be  racemose.  In  many  glands  the  secreting  epi- 
thelium is  confined  to  the  deeper  part  of  the  tube,  the 
alveolus  or  acinus  (Fig.  6),  while  the  more  superficial  part 
is  lined  by  cells  which  do  not  secrete,  forming  the  duct. 

In  many  situations  several  simple  glands  are  grouped 
together,  their  ducts  opening  into  one  common  duct,  and 
a  compound  gland  results. 

Secreting  epithelium  varies  according  to  the  material  it 
produces. 

(A)  Mucin-secreting  Epithelium. — Many  glands  have  for 
their  function  the  production  of  mucin,  a  slimy  substance 
of  use  in  lubricating  the  mouth,  stomach,  intestine,  &c. 
The  acini  containing  such  cells  are  usually  large.  The  cells 
themselves  are  large,  and  are  placed  on  a  delicate  basement 
membrane,  a  condensation  of  the  subjacent  fibrous  tissue, 
which  bounds  the  acinus.  The  nuclei  are  situated  near 
to  the  attached  margin  of  the  cells,  which  are  somewhat 
irregular  in  shape,  and  are  packed  close  together.  Their 
appearance  varies  according  to  whether  the  gland  has  been 
at  rest  or  has  been  actively  secreting. 

Resting  State. — In  the  former  case,  in  the  fresh  condi- 
tion, the  cells  are  large,  and  pressed  closely  together.  Their 
protoplasm  is  filled  with  large  shining  granules.  After  treat- 
ment with  reagents,  each  cell  becomes  distended  with  clear, 
transparent  mucin,  formed  by  the  swelling  and  coalescence 
of  the  granules. 

After  Activity. — After  the  gland  has  been  actively  secret- 
ing, the  cells  are  smaller  and  the  granules  are  much  less 
numerous,  being  chiefly  situated  at  the  free  extremity  of  the 
cell,  and  leaving  the  nucleus  much  more  apparent. 

This  form  of  epithelium,  during  the  resting  condition  of 


26  HUMAN   PHYSIOLOGY 

the  gland,  takes  up  nourishing  matter  and  forms  this  mucin- 
yielding  substance.  During  the  active  state  of  the  gland, 
the  mucin-yielder  is  changed  to  mucin,  and  is  extruded  from 
the  cells  into  the  lumen  of  the  gland. 

Mucin  is  a  substance  of  great  importance  in  the  animal 
economy.  When  precipitated  and  freed  from  water  it  is 
white  and  amorphous.  On  the  addition  of  water  it  swells 
up  and  forms  a  glairy  mass.  In  the  presence  of  alkalies,  it 
forms  a  more  or  less  viscous  solution,  and  from  this  solution 
it  is  precipitated  by  acetic  acid.  In  composition  it  is  a  pro- 
teid  linked  to  a  molecule  allied  to  the  sugars,  glucosamine 
C6H11NH205,  and  is  therefore  called  a  glyco-proteid.  When 
boiled  with  an  acid  it  yields  sugar.  (See  Chemical  Physiology, 
p.  6.) 

In  adult  life  the  great  function  of  mucin  is  to  give  to 
certain  secretions  a  slimy  character  which  renders  them  of 
value  as  lubricants. 

(B)  Zymin-secreting  Epithelium. — Another  form  of  secret- 
ing epithelium  of  great  importance  is  that  which  forms  the 
various  juices  which  act  upon  the  food  to  digest  it.  These 
juices  owe  their  activity  to  the  presence  of  enzymes  or  zymins. 

A  zymin-forming  gland  after  a  prolonged  period  of  rest 
shows  cells  closely  packed  together,  so  that  it  is  difficult  to 
make  out  their  borders.  The  protoplasm  is  loaded  with 
granules  which  are  much  smaller  than  those  seen  in  the 
mucin-forming  cells,  and  which  do  not  swell  up  in  the  same 
way,  under  the  action  of  reagents.  The  nucleus  is  often 
obscured  by  the  presence  of  these  granules. 

When  the  gland  has  been  actively  secreting,  the  granules 
become  fewer  in  number,  and  are  confined  to  the  free  ex- 
tremity of  the  cell;  they  are  obviously  passing  out.  The 
cell  becomes  smaller,  and  its  outlines  are  more  distinct  and 
the  nucleus  more  apparent. 

The  granules  which  fill  the  cells  are  not  composed  of  the 
active  enzyme.  If  extracts  of  the  living  cells  be  made,  they 
are  inert,  and  it  is  only  after  the  granules  have  left  the  cell, 
or  are  in  the  process  of  leaving,  that  they  become  active. 
Hence,  the  granules  are  said  to  be  composed  of  zymin- 
forming  substance  or  zymogen. 

The  series  of  changes  are  parallel  to  those  described  in  the 


THE   TISSUES  27 

mucin-forming  cells.  During  the  so-called  resting  state  of 
the  gland,  the  cells  are  building  up  zymogen.  When  the 
gland  is  active,  the  cells  throw  off  the  material  they  have 
accumulated,  and  it. undergoes  a  change  to  zymin. 

(C)  Excreting  Epithelium  does  not  manufacture  materials 
of  use  in  the  animal  economy,  but  passes  substances  out  of 
the  body.  Such  epithelium  is  seen  in  the  kidneys,  sweat 
glands,  sebaceous  glands,  mammary  glands,  and  perhaps  in 
the  liver.  The  cells  are  composed  of  a  granular  protoplasm, 
in  which  the  presence  of  the  material  to  be  excreted  either 
in  its  fully  elaborated  condition,  or  in  process  of  preparation, 
may  frequently  be  demonstrated — e.g.  fat  globules,  iron  con- 
taining particles,  &c.  These  cells  do  not  merely  take  up 
material  from  the  blood  and  pass  it  out,  but  they  may  pro- 
foundly alter  it  before  getting  rid  of  it. 

4.  Ciliated  Epithelium  (Fig.  5,  p.  24). — The  cells  are 
usually  more  or  less  columnar,  and  the  free  border  is  pro- 
vided with  a  series  of  hair-like  processes,  the  cilia,  which 
vary  in  size  in  different  situations. 

In  the  living  state  the  cilia  are  in  constant  rhythmic 
motion,  each  cilium  being  suddenly  whipped  or  bent  down 
in  one  direction,  and  then  again  assuming  the  erect  position. 

All  the  cilia  on  a  surface  work  harmoniously  in  the  same 
direction,  and  the  movement  passes  from  the  cilia  of  one  cell 
to  those  of  the  next  in  regular  order,  beginning  at  one  end 
of  the  surface  and  passing  to  the  other. 

As  a  result  of  this  constant  harmonious  rhythmic  move- 
ment, any  matter  lying  upon  the  surface  is  steadily  whipped 
along  it;  and  since  the  cilia  usually  work  from  the  inner 
parts  of  the  body  to  the  outside,  this  matter  is  finally  ex- 
pelled from  the  body. 

The  movements  of  the  cilia  are  dependent  on  the  changes 
in  the  protoplasm,  and  everything  which  influences  the  rate 
of  chemical  change  modifies  the  rate  of  ciliary  movement, 
which  may  thus  be  taken  as  an  index  of  the  protoplasmic 
activity. 

II.  CONNECTIVE   TISSUES 

1.  Mucoid  Tissue.  —  The  cells  of  the  mesoblast  of  the 
embryo,  which  at  first  lie  in  close  apposition  with  one 


28 


HUMAN   PHYSIOLOGY 


another,  become  separated,  remaining  attached  by  elongated 
processes.  Between  the  cells,  a  clear,  transparent,  viscous 
mucin-like  substance  makes  its  appearance,  forming  a  soft 
jelly-like  tissue.  This  tissue  is  widely  distributed  in  the 
embryo  as  a  precursor  of  the  connective  tissues,  and  after 
birth  it  is  still  to  be  seen  in  the  pulp  of  a  developing  tooth 
and  in  the  vitreous  humour  of  the  eye  (Fig.  7). 


FlO.  7.— Mucoid  Tissue  from  an  embryo  rabbit. 

2.  Fibrous  Tissue. — As  development  advances,  the  cells  of 

mucoid  tissue  elongate 
and  become  spindle- 
shaped,  and  are  continued 
at  their  ends  into  fibres 
(Fig.  8).  These  cells  are 
often  called  fibroblaats. 
The  connective  tissues  are 
thus  clearly  distinguished 
from  the  epithelia  byhav- 

Fio.S.-Fibroblasts  from  young  fibrous  -          the   forme(i    material 

tissue.  , 

between  and  not  in  the 

cells.     They  are  composed  of  the  following  parts : — 
I.  Formed  material. 
(a)  Fibres. 
(6)  Matrix. 

II.  Spaces  (Connective  Tissue  Spaces). 
III.  Cells. 


THE  TISSUES 


29 


I.  Formed  Material.  —  (A)  Fibres  (Fig.  9)— 1st,  Non- 
elastic  (white  fibres).  These  are  delicate,  transparent  fibrils 
arranged  in  bundles,  which  do  not  branch,  and  which  have 
a  mucin-like  matrix  between  them.  They  are  composed  of 
a  non-elastic  substance,  collagen.  This  is  closely  allied  to 
the  proteids,  and  gives  the  proteid  reactions  faintly,  but  it 
does  not  yield  tyrosin  when  decomposed,  while  it  does  yield 
amido-acetic  acid.  It  is  insoluble  in  cold  water,  but  swells 
up  and  becomes  transparent  in  acetic  acid.  It  has  a  great 
affinity  for  carmine,  and  stains  a  pink  colour  with  it.  When 
boiled,  it  takes  up  water  to  form  a  hydrate,  Gelatin ;  a  sub- 


FIG.  9.— Bundles  of  White  Fibres,  with  Fibroblasts  (a)  and  Elastic  Fibres  (6) 
anastomosing  with  one  another. 

stance  soluble  in  hot  water,  and  forming  a  jelly  on  cooling. 
(See  Chemical  Physiology,  p.  7.) 

2nd,  Elastic  Fibres.  These  are  highly  refractile  elastic 
fibres,  which  branch  and  anastomose  with  one  another. 
They  are  composed  of  Elastin,  a  near  ally  of  the  proteids, 
which  is  insoluble  both  in  cold  and  in  hot  water,  and  is  not 
acted  on  by  acetic  acid.  It  stains  yellow  with  picric  acid, 
and  has  no  affinity  for  carmine. 

(B)  Interstitial  Substance. — This  is  composed  of  mucus- 
like  material. 

Types  of  Fibrous  Tissue. — According  to  the  arrangement 
of  these  fibres,  and  to  the  preponderance  of  one  or  other 
variety,  various  types  of  fibrous  tissue  are  produced. 


3o  HUMAN   PHYSIOLOGY 

When  a  padding  is  required,  as  under  the  skin  and  under 
mucous  membranes,  the  fibres  are  arranged  in  a  loose  felt 
work  to  constitute  Areolar  Tissue. 

In  fascia,  in  tendon  sheaths,  and  in  flat  tendons,  the  fibres 
are  closely  packed  together  to  form  more  or  less  definite 
layers.  In  tendons  and  ligaments  the  fibres  run  parallel 
and  close  together.  In  ordinary  tendons,  where  no  elasticity 
is  required,  the  fibres  are  of  the  white  or  non-elastic  variety. 
In  ligaments,  where  elasticity  is  desirable,  the  elastic  fibres 
preponderate. 

Lymph  Tissue. — One  peculiar  modification  of  fibrous  tissue 
is  often  described  as  a  special  tissue  under  the  name  of  Lymph 
Tissue.  It  is  composed  of  a  delicate  network  of  white  fibres, 
the  interstices  of  which  communicate  with  lymphatic  vessels, 
and  contain  masses  of  simple  protoplasmic  cells,  leucocytes, 
often  in  a  state  of  active  division.  So  numerous  are  these 
that  it  is  impossible  to  make  out  the  network  under  the 
microscope,  until  they  have  been  removed  by  washing. 

Lymph  tissue  is  very  widely  distributed  throughout  the 
body,  and  is  of  great  importance  in  connection  with  nutrition. 

II.  The  spaces  of  fibrous  tissue  vary  with  the  arrangement 
of  the  fibres.     In  the  loose  areolar  tissue  under  the  skin 
they  are  very  large  and  irregular,  in  fascia  they  are  flattened, 
while  in  tendon,  where  the  fibres  are  in  parallel  bundles,  they 
are  long  channels. 

III.  The  cells  of  fibrous  tissue  (Fibroblasts)  vary  greatly 
in  shape.     In  the  young  tissue  they  are  elongated  spindles, 
from  the  ends  of  which  the  fibres  extend.     In  some  of  the 
loose  fibrous  tissues  they  retain  this  shape,  but  in  the  denser 
tissues  they  get  squeezed  upon,  and  are  apt  to  be  flattened 
and  to  develop  processes  thrust  out  into  the  spaces. 

In  certain  situations,  peculiar  modifications  of  Connective 
Tissue  cells  are  to  be  found — 

(A)  Endothelium. — When  cells  line  the  larger  connective 
tissue  spaces  they  become  flattened,  and  form  a  covering 
membrane,  called  an  endothelium.  Such  a  layer  lines  all 
the  serous  cavities  of  the  body,  and  the  lymphatics,  blood 
vessels,  and  heart,  which  are  all  primarily  large  connective 
tissue  spaces.  To  demonstrate  the  outlines  of  these  cells  it 
is  necessary  to  stain  with  nitrate  of  silver,  which  has  a  special 


THE  TISSUES  31 

affinity  for  the  interstitial  substance,  and  which  thus  forms 
a  series  of  black  lines  between  the  cells. 

(B)  Fat  Cells. — In  the  areolar  tissue  of  many  parts  of  the 
body  fat  makes  its  appearance  in  the  cells  round  the  smaller 
blood  vessels,  and  when  these  cells  occur  in  masses  Adipose 
Tissue  is  produced. 

Little  droplets  of  oil  first  appear,  and  these  become  larger, 
run  together,  and  finally  form  a  large  single  globule,  distend- 
ing the  cell,  and  pushing  to  the  sides  the  protoplasm  and 
nucleus  as  a  sort  of  capsule  (Fig.  10). 


FIG.  10. — Fat  Cells  stained  with  osmic  acid,  and  lying  alongside 
a  small  blood  vessel. 

If  the  animal  be  starved,  the  fat  gradually  disappears  out 

of  the  cell,  and  in  its  place  is  left  a  clear  albuminous  fluid 

which  also  disappears,  and  the  cell  resumes  its  former  shape. 

The  fats  are  ether  derivatives   usually  of  the  triatomic 

alcohol  glycerin — 

(OH 

C3H5    OH 
(OH 

by  the  replacement  of  the  hydrogen  of  the  hydroxyl  molecules 
by  the  radicles  of  the  fatty  acids. 

The  most  abundant  fatty  acids  of  the  body  are : — 

Palmitic  Acid,  C16H31O,OH 

Stearic  Acid,  C18H350,OH 

Oleic  Acid,  C18H330,OH 


32  HUMAN  PHYSIOLOGY 

and  from  these  the  three  fats — 

Palmitin  C3H5(0,C18H3I0)3  -  C51H9806 

Stearin 

Olein 

are  produced. 

It  will  be  observed  that  the  molecules  of  these  fats  are 
very  rich  in  carbon  and  hydrogen,  and  very  poor  in  oxygen 
— i.e.  they  contain  a  large  amount  of  material  capable  of 
being  oxidised,  and  thus  capable  of  affording  energy  in  the 
process  of  combustion. 

The  fats  resemble  one  another  in  being  insoluble  in  water, 
but  soluble  in  ether  and  in  hot  alcohol.  As  the  alcohol 
cools,  they  separate  out  as  crystals.  They  differ  from  one 
another  in  their  melting  point,  palmitin  melting  at  the 
highest  and  olein  at  the  lowest  temperature.  Fat  which  is 
rich  in  palmitin  and  stearin,  like  ox  fat,  is  thus  hard  and 
solid  at  the  ordinary  temperature  of  the  air,  while  fats  rich 
in  olein,  like  dogs'  fats,  are  semi-fluid  at  the  same  tempera- 
ture. The  olein  acts  as  a  solvent  for  the  fats  of  a  higher 
melting  point.  (For  tests,  see  Chemical  Physiology,  p.  12.) 

The  functions  of  adipose  tissue  are  twofold  : — 

1st,  Mechanical. — The  mass  of  adipose  tissue  under  the 
skin  is  of  importance  in  protecting  the  deeper  structures 
from  injury.  It  is  a  cushion  on  which  external  violence 
expends  itself.  Further,  this  layer  of  subcutaneous  fat  pre- 
vents the  loss  of  heat  from  the  body,  being,  in  fact,  an  extra 
garment. 

2nd,  Chemical. — Fat,  on  account  of  its  great  quantity  of 
unoxidised  carbon  and  hydrogen,  is  the  great  storehouse  of 
energy  in  the  body  (p.  379). 

(C)  Pigment  Cells. — In  various  parts  of  the  eye  the 
connective  tissue  and  other  cells  contain  a  black  pigment — 
Melanin.  The  precise  chemistry  and  mode  of  origin  of  this 
pigment  is  not  known.  It  contains  carbon,  hydrogen,  nitro- 
gen, oxygen,  and  it  may  also  contain  iron.  It  may  be  formed 
directly  in  the  cells,  or  it  may  be  produced  by  the  cells  from 
the  pigment  of  the  blood.  Its  function  in  the  eye  is  to 
prevent  the  passage  of  light  through  the  tissues  in  which  it 
is  contained. 


THE  TISSUES 


33 


The  cells  containing  the  pigment  are  branched,  and  in 
many  cases  they  possess  the  power  of  movement.  This  is 
specially  well  seen  in  such  cells  in  the  skin  of  the  frog,  where 
contraction  and  expansion  may  be  easily  studied  under  the 
microscope.  By  these  movements  the  skin,  as  a  whole,  is 
made  lighter  or  darker  in  colour.  The  movements  of  these 
cells  are  under  the  control  of  the  central  nervous  system. 

3.  Cartilage. — While  fibrous  tissue  is  the  great  binding 
medium  of  the  body,  support  is  afforded  in  foetal  life  and  in 
certain  situations  in  adult  life  by  cartilage. 

When  cartilage  is  to  be  formed,  the  embryonic  cells  be- 
come more  or  less  oval,  and  secrete  around  them  a  clear 
pellucid  capsule.  This  may  become  hard,  and  persist 
through  life  as  in  the  so-called  parenchymatous  cartilage 
of  the  mouse's  ear. 

(1)  Hyaline  Cartilage. — Development,  however,  usually 
goes  further,  and  before  the  capsule  has  hardened,  the  carti- 
lage cells  again  divide,  and  each 
half  forms  a  new  capsule  which 
expands  the  original  capsule  of 
the  mother  cell,  and  thus  in- 
creases the  amount  of  the 
formed  material.  This  formed 
material  has  a  homogeneous, 
translucent  appearance,  and  a 
tough  and  elastic  consistence, 
and  cuts  like  cheese  with  the 
knife  (Fig.  11). 

The  formed  material  of  car- 
tilage is  not  a  special  substance, 
but  a  mixture  of  chondroitin- 
sulphuric  acid  with  collagen  in 
combination  with  proteids.  Chondroitin  when  decomposed 
yields  glucosamine,  a  sugar-like  substance  containing  nitrogen, 
and  glycuronic  acid,  another  substance  closely  related  to  the 
sugars. 

Cartilage  is  surrounded  by  a  fibrous  membrane,  the  peri- 
chondrium,  and  no  hard  and  fast  line  of  demarcation  can 
be  made  out  between  them.  The  fibrous  tissue  gradually 
becomes  less  fibrillated — the  cells  become  less  elongated  and 

3 


FIG.  11. — Hyaline  Cartilage  covered 
by  perichondrium. 


34  HUMAN  PHYSIOLOGY 

more  oval,  as  if  the  interfibrillar  substance  increased  in 
amount  and  became  of  the  same  refractive  index  as  the 
fibres.  During  old  age,  a  fibrillation  of  the  homogeneous- 
looking  cartilage  is  brought  out,  especially  in  costal  cartilage, 
by  the  deposition  of  lime  salts  in  the  matrix,  between  the 
fibres.  It  was  long  ago  shown  that  in  inflammation  of 
cartilage  this  fibrillation  appears ;  and  by  digesting  in  baryta 
water,  a  similar  structure  may  be  brought  out.  The  close 
connection  of  cartilage  with  fibrous  tissue  is  thus  clearly 
demonstrated. 

Such  homogeneous  or  hyaline  cartilage  precedes  most  of 
the  bones  in  the  embryo,  and  covers  the  ends  of  the  long 
bones  in  the  adult  (articular  cartilage),  forms  the  frame- 
work of  the  larynx  and  trachea,  and  constitutes  the  costal 
cartilages. 

(2)  Elastic    Fibro-Cartilage. — In     certain     situations,     a 
specially  elastic  form  of  cartilage  is  developed — e.g.  in  the 
external  ear,   elastic   fibres   appearing   in  the  cartilaginous 
matrix,  and  forming  a  network  through  it. 

(3)  White  Fibro-Cartilage. — In  other  situations — e.g.  the 
intervertebral  discs — a  combination  of  the  binding  action  of 
fibrous  tissue  with  the  padding  action  of  cartilage  is  required ; 
and  here  strands  of  white  fibrous  tissue  with  little  islands  of 
hyaline  cartilage  are   found.     It  is  also  found  when  white 
fibrous  tissue,  as  tendon,  is  inserted  into  hyaline  cartilage, 
and  is  really  a  mixture  of  two  tissues — white  fibrous  tissue 
and  cartilage. 

4.  Bone. — The  great  supporting  tissue  of  the  adult  is  BONE. 

(1)  DEVELOPMENT  AND  STRUCTURE. — Bone  is  formed  by  a 
deposition  of  lime  salts  in  layers  or  lamellae  of  white  fibrous 
tissue ;  but  while  some  bones,  as  those  of  the  cranial  vault, 
face,  and  clavicle,  are  produced  entirely  in  fibrous  tissue, 
others  are  performed  in  cartilage,  which  acts  as  a  scaffolding 
upon  which  the  formation  of  bone  goes  on. 

Intra-membranous  Bone  Development. — This  may  be  well 
studied  in  any  of  the  bones  of  the  cranial  vault  where 
cartilage  is  absent  (Fig.  12). 

At  the  centre  of  ossification  the  matrix  between  the  fibres 
becomes  impregnated  with  lime  salts,  chiefly  the  phosphate 
and  carbonate.  How  this  deposition  takes  place  is  not 


THE   TISSUES 


35 


known ;  and  how  far  it  is  dependent  on  the  action  of  cells 
has  not  been  clearly  determined.     As  a  result  of  this,  the 
connective    tissue    cells    get    enclosed   in   definite    spaces, 
lacunae,   and   become   bone  corpuscles.     Narrow   branching 
channels  of  communication 
are  left  between  theselacunae, 
the  canaliculi.    This  deposi- 
tion of  lime  salts  spreads  out 
irregularly  from  the  centre 

into    the    adjacent    fibrous  **%* 

tissue.  The  fully  formed 
adult  bone,  however,  is  not 
a  solid  block,  but  is  com- 
posed of  a  compact  tissue 
outside,  and  of  a  spongy  bony 
tissue,  cancellous  tissue,  in- 
side. This  cancellous  tissue 
is  formed  as  a  secondary 
process.  Into  the  block  of  ^ 

calcareous  matter,  formed  HHHI 
as  above  described,  processes 
of  the  fibrous  tissue,  with 
blood  vessels,  lymphatics, 
and  numerous  cells,  burrow. 
This  burrowing  process 
seems  to  be  carried  on  by 
the  connective  tissue  cells 

which  eat  up  the  bony  matter  formed.  In  doing  this  they 
frequently  change  their  appearance,  becoming  large  and 
multi-nucleated.  Thus  the  centre  of  the  bone  is  eaten  out 
into  a  series  of  channels,  in  which  the  marrow  of  the  bone  is 
lodged,  and  between  which  narrow  bridges  of  bone  remain. 

It  is  by  the  extension  of  the  calcifying  process  outwards, 
and  the  burrowing  out  of  the  central  part  of  the  bone,  that 
the  diploe  and  cancellous  tissue  are  produced. 

Intra-cartilaginous  Bone  Development. — In  the  bones 
preformed  in  cartilage,  the  process  is  somewhat  more  com- 
plex, although  all  the  bone  is  formed  in  connection  with 
fibrous  tissue,  the  cartilage  merely  playing  the  part  of  a 
scaffolding  and  being  all  removed.  Where  the  adult  bone 


FIG.  12. — Intra  -  membranous  Bone 
Development  in  the  lower  jaw  of  a 
foetal  cat.  Above,  the  process  of 
ossification  is  seen  shooting  out 
along  the  fibres,  and  on  the  lower 
surface  the  process  of  absorption  is 
going  on.  Three  osteoclasts — large 
multi-nucleated  cells — are  shown. 


36  HUMAN  PHYSIOLOGY 

is  to  be  produced,  a  minute  model  is  formed  in  hyaline  car- 
tilage in  the  embryo,  and  this  is  surrounded  by  a  fibrous 
covering,  the  perichondrium.  In  the  deepest  layers  of  this 
perichondrium  the  process  of  calcification  takes  place  as 
described  above,  and  spreads  outwards,  thus  encasing  the 
cartilage  in  an  ever  thickening  layer  of  bone  (Fig.  13).  This 
was  demonstrated  by  inserting  a  silver  plate  under  the  peri- 
osteum, and  showing  that  bone  was  deposited  outside  of  it. 


Flo.  13. — Intra-cartilaginous  Bone  Development.  A  phalanx  of  a  foetal  finger 
showing  the  formation  of  periosteal  bone  round  the  shaft ;  the  opening  up 
of  the  cartilage  at  the  centre  of  ossification  ;  the  vascularisation  of  the 
cartilage  by  the  invasion  of  periosteum  ;  and  the  calcification  of  the  carti- 
lage round  the  .spaces. 

At  the  same  time,  in  the  centre  of  the  cartilage,  at  what 
is  called  the  centre  of  ossification,  the  cells  begin  to  divide 
actively,  and,  instead  of  forming  new  cartilage,  eat  away 
their  capsules,  and  thus  open  out  the  cartilage  spaces 
(Fig.  13).  Into  these  spaces  processes  of  the  perichondrium 
bore  their  way,  carrying  with  them  blood  vessels,  and  thus 
rendering  the  cartilage  vascular  (Fig.  13).  The  vas- 
cularisation of  the  centre  of  the  cartilage  having  been 
effected,  the  process  of  absorption  extends  towards  the 
two  ends  of  the  shaft  of  cartilage,  which  continues  to 
elongate.  The  cartilage  cells  divide  and  again  divide,  and, 
by  absorbing  the  material  between  them,  form  long  irregular 
canals  running  in  the  long  axis  of  the  bone,  with  trabeculse 


THE  TISSUES 


37 


of  cartilage  between  them.  Into  these  canals  the  processes 
of  the  periosteum  extend,  and  fill  them  with  its  fibrous 
tissue.  In  this  fibrous  tissue,  the  deposition  of  lime  salts 
takes  place  upon  the  trabeculae,  enclosing  cells,  and  thus 
forming  a  crust  of  bone,  while  the  cartilage  also  becomes 
calcified.  If  this  calcification  of  the  cartilage  and  deposition 
of  bone  were  to  go  on  unchecked,  the  block  of  cartilage 
would  soon  be  reduced  to  a  solid  mass  of  calcified  tissue. 
But  this  does  not  occur.  For,  as  rapidly  as  the  trabeculse 
become  calcified,  they  are 
absorbed,  while  the  active 
changes  extend  farther  and 
farther  from  the  centre,  which 
is  thus  reduced  to  a  space 
filled  by  fibrous  tissue  which 
afterwards  becomes  the  bone 
marrow. 

The  process  of  absorption 
does  not  stop  at  the  original 
block  of  cartilage,  which  is 
all  removed.  After  all  of  this 
has  been  absorbed,  the  bone 
formed  round  the  cartilage  (the 
periosteal  bone)  is  attacked  by 
burrowing  processes  from  in- 
side and  outside,  which  hollow 
out  long  channels  running  in 
the  long  axis  of  the  bone. 
These  are  -the  Haversian 
spaces  (Fig.  14).  Round  the 
inside  of  these,  calcification 

occurs,  spreading  inwards  in  layers,  and  enclosing  connective 
tissue  cells,  until,  at  length,  only  a  small  canal  is  left,  an 
Haversian  canal,  containing  some  connective  tissue,  blood 
vessels,  lymphatics,  and  nerves,  with  layer  upon  layer  of 
bone  concentrically  arranged  around  it.  This  constitutes  an 
Haversian  system.  In  this  way  the  characteristic  appearance 
of  the  shaft  of  a  long  bone  is  produced  (Fig.  14),  with  layers 
of  calcified  fibrous  tissue,  the  bone  lamellae  arranged  as  shown 
as  Haversian,  interstitial,  peripheral,  and  medullary  lamellae. 


FIG.  14. — Cross  Section  through  part 
of  the  shaft  of  an  adult  long  bone  to 
show  the  arrangement  in  lamellae 
distributed  as  Haversian  (1),  inter- 
stitial (2),  peripheral  (3),  and  medul- 
lary (4). 


38  HUMAN  PHYSIOLOGY 

One  important  function  performed  by  the  cartilage  is  in 
bringing  about  the  increase  in  length  of  the  bones.  In 
addition  to  the  centre  of  ossification  in  the  shaft,  at  each 
end  of  the  bone  one  or  more  similar  centres  of  ossification 
form.  These  are  the  epiphyses.  Between  these  and  the 
central  rod  of  bone — the  diaphysis — a  zone  of  cartilage 
exists  until  near  adult  life,  when  the  bones  stop  growing. 
In  this  zone,  the  cells  arrange  themselves  in  vertical  rows, 
divide  at  right  angle  to  the  long  axis  of  the  bone,  and  form 
cartilage.  This  cartilage  as  it  is  formed  is  attacked  by  the 
bone-forming  changes  at  the  diaphysis  and  epiphyses,  but 
the  amount  of  new  cartilage  formed  is  proportionate  to  this, 
and  thus  a  zone  of  growing  cartilage  continues  to  exist  until 
early  adult  life,  when  epiphyses  and  diaphysis  join  and 
growth  in  length  is  stopped.  The  rate  and  extent  of  this 
growth  of  the  cartilage  has  an  important  influence  on  the 
growth  of  the  individual. 

(2)  CHEMISTRY. — The  composition  of  adult  bone  is  roughly 
as  follows : — 

Water,  10  per  cent. 
Solids,  90  per  cent. 

Organic,  35  per  cent. — chiefly  collagen. 
Inorganic,  65  per  cent. 
Calcium  phosphate,  51. 
„         carbonate,  11. 

fluoride,  0*2. 

Magnesium  phosphate,  1. 
Sodium  salts,  1. 

The  points  to  be  remembered  are  the  small  amount  of 
water,  the  large  amount  of  inorganic  matter,  chiefly  calcic 
phosphate,  and  the  nature  of  the  organic  matter — collagen. 


(B)  THE   MASTER   TISSUES   OF   THE    BODY, 
MUSCLE   AND   NERYE. 

By  means  of  the  epithelial  and  connective  tissues  the 
body  is  protected,  supported,  and  nourished.  It  performs 
purely  vegetative  functions,  but  it  is  not  brought  into 
relationship  with  its  environments.  By  the  development 


THE   TISSUES  39 

of  nerve  and  muscle  the  surroundings  are  able  to  act  upon 
the  body,  and  the  body  can  react  upon  its  surroundings. 

These  tissues  may  thus  be  called  the  Master  Tissues, 
and  it  is  as  the  servants  of  these  tissues  that  all  the  others 
functionate. 

So  far  as  the  chemical  changes  in  the  body  are  concerned, 
muscle  is  more  important  than  nerve,  for  three  reasons — 
First,  it  is  far  more  bulky,  making  up  something  like  42 
per  cent,  of  the  total  weight  of  the  body  in  man ;  second, 
it  is  constantly  active,  for  even  in  sleep  the  muscles  of 
respiration,  circulation,  and  digestion  do  not  rest ;  and  third, 
the  changes  going  on  in  it  are  very  extensive,  since  its  great 
function  is  to  set  free  energy  from  the  food.  So  far  as  the 
metabolism  of  the  body  is  concerned,  muscle  is  the  master 
tissue.  For  muscle  we  take  food  and  breath,  and  to  get 
rid  of  the  waste  of  muscle  the  organs  of  excretion  act. 
Hence  it  is  in  connection  with  muscle  that  all  the  problems 
of  nutrition — digestion,  respiration,  circulation,  and  excretion 
— -have  to  be  studied. 

I.   MUSCLE. 

The  two  great  functions  of  muscle  are — 

To  perform  mechanical  work. 

To  liberate  heat. 

The  study  of  the  physiology  of  muscle  may  be  divided 
into — 

1.  The   structure,   chemical   and   physical   characters   of 
muscle  at  rest. 

2.  The  methods  of  making  muscle  contract. 

3.  The  changes  which  take  place  in  muscle  during  con- 
traction. 

4.  The  chemical  change  in  muscle  and  the  source  of  the 
energy  evolved. 

5.  Death  of  muscle. 

1.  STRUCTURE,  CHEMICAL  AND  PHYSICAL  CHARACTERS  OF 
MUSCLE  AT  REST. 

The  first  trace  of  the  evolution  of  muscle  is  found  among 
the  infusoria,  where,  in  certain  cells,  in  parts  of  the  proto- 
plasm, the  network  or  cytomitoma  is  arranged  in  long  parallel 


40  HUMAN   PHYSIOLOGY 

threads   in   the  direction  of  which  the  cell  contracts  and 
expands.     Such  a  development  has  been  termed  a  myoid. 

i.  Structure  of  Muscle. 

Even  a  cursory  examination  of  mammalian  muscles  shows 
that  those  of  the  trunk  and  limbs,  skeletal  muscles,  are 
different  from  those  of  such  internal  organs  as  the  bladder, 
uterus  and  alimentary  canal,  visceral  muscles. 

The  visceral  muscles  appear  to  be  formed  from  cells 
similar  to  ordinary  connective  tissue  cells.  These  elongate, 
acquire  a  covering,  and  their  protoplasm  becomes  definitely 
longitudinally  fibrillated  by  the  arrangement  of  the  cytomi- 
torna.  They  thus  become  spindle-shaped  cells,  varying 


FIG.  15.— (a)  Fibres  of  Visceral  Muscle ;  (b)  Fibres  of  Skeletal  Muscle 
to  show  sarcolemma,  muscle  corpuscles,  and  sarcous  substance 
composed  of  fibrils  showing  transverse  markings. 

in  length  from  about  50  to  200  micro-millimetres.  The 
covering  membrane,  sarcolemma,  is  thin,  but  tough  and 
elastic,  and  adapts  itself  to  the  surface  of  the  cell,  unless 
when  this  is  excessively  shortened,  in  which  case  the 
sarcolemma  may  be  thrown  into  folds,  which  give  the  cell 
the  appearance  of  cross-striping.  The  nucleus  is  usually 
long,  almost  rod-shaped,  and  is  independent  of  the  cytomi- 
toma  (Fig.  15,  a). 

The  skeletal  muscles  develop  from  a  special  set  of  cells, 
early  differentiated  as  the  muscle  plates  in  the  mesoblast 
down  each  side  of  the  vertebral  column  of  the  embryo.  Each 
cell  elongates.  The  nucleus  divides  across,  but  the  cell, 
instead  of  also  dividing,  lengthens  and  continues  to  elongate 
as  the  two  daughter  nuclei  again  divide.  The  cytomitoma 
becomes  arranged  longitudinally,  and  a  series  of  transverse 


THE  TISSUES  41 

markings  appear  across  the  cell.     Lastly,  a  covering  develops, 
and  the  fully-formed  fibre  is  produced  (Fig.  15,  6). 
This  consists  of  three  parts — 

1.  The  Sarcolemma  is  a  delicate,  tough,  elastic  membrane 
closely  investing  the  fibre,  and  attached   to  it  at   Dobie's 
lines. 

2.  The    Muscle    corpuscles    consist   of  little    masses    of 
protoplasm  with  a  nucleus,  which  lie  just  under  the  sarco- 
lemma. 

3.  The   Sarcous   substance   is   made   up   of   a    series   of 
longitudinal  fibrils  consisting  of  alternate  dim  and  clear  bands 
— the  former  staining  deeply  with  eosin.     In  the  middle 
of  the  clear  band  is  a  narrow  dim  line,  Dobie's  line.     The 
fibres  and  fibrils  tend  to  break  across  in  the  region  of  the 
clear  band,  showing  that  they  are  weakest  at  that  part.    The 
clear  band  differs  from  the  dim  band,  not  only  in  not  taking 
up  eosin,  but  also  in  the  fact  that  it  entirely  prevents  the 
passage   of  polarised   light   except  in  one  position   of  the 
analysing  prism,  while  the  dim  band  allows  polarised  light  to 
pass,  whatever  be  the  position  of  the  prisms. 

Two  explanations  of  these  facts  have  been  suggested: 
(1)  That  the  sarcous  substance  is  made  up,  like  other  proto- 
plasm, of  a  mitoma  and  plasma  ;  that  the  mitoma  is  arranged 
in  a  series  of  longitudinal  fibres,  which  are  broader  and 
stronger  in  the  dim  band,  and  lie  closely  applied  to  one 
another,  side  by  side,  while  in  the  clear  band  they  are 
thinner,  and  are  separated  from  one  another  by  plasma ;  and 
that  at  Dobie's  line  there  is  a  swelling  on  each  fibril ;  (2) 
thai  each  fibril  is  a  hollow  tube  consisting  of  a  sponge-work 
in  the  dim  band  with  a  fluid  part  in  the  clear  band  inter- 
sected by  a  septum  at  Dobie's  line. 

2.  Chemistry  of  Muscle. 

Like  all  other  living  tissues,  muscle  is  largely  composed  of 
water.  It  contains  about  75  per  cent.  The  25  per  cent, 
of  solid  constituents  is  made  up  of  a  small  quantity,  about  3 
per  cent.,  of  ash,  and  21  per  cent,  of  organic  substances. 
The  ash  consists  chiefly  of  potassium  and  phosphoric  acid, 
with  small  amount  of  hydrochloric  acid  and  sodium,  mag- 


42  HUMAN   PHYSIOLOGY 

nesium,  calcium,  and  iron.  But  a  part  of  the  phosphoric 
acid  is  derived  from  the  phosphorus  of  the  nuclei  of 
muscle,  and  probably  from  phosphocarnic  acid. 

1.  Proteids. — Of    the   organic    constituents,    by   far    the 
greater  part  is  made  up  of  Proteids.     These  may  be  divided 
into — 

(a)  Those  soluble  in  neutral  salt  solutions. 
(6)  Those  insoluble  in  them. 

(a)  The   first   class   of    bodies  consist   entirely   of    three 
globulins.     Two  of  these — Myosinogen  and  Paramyosinogen 
—have  the  peculiar  property  of  clotting  under  certain  condi- 
tions, of  forming  what  is  called  Myosin,  and  this  process, 
which   occurs   after  death,   is   the   cause   of  death   stiffen- 
ing.    The  post-mortem  change  is  supposed  to  be  brought 
about  by  the  development  of  an  enzyme,  since  a  glycerine 
extract   of  dried   muscle  rapidly  causes  the   formation   of 
myosin.     The  third  globulin,  Myoglobulin,  does  not  undergo 
this   change.      These  three   proteids   are   contained   in   the 
plasma — the  juice  which  can  be  expressed  from  muscles  kept 
near  the  freezing  point.     If  the  plasma  is  warmed  it  rapidly 
forms  a  jelly,  clots  just  as  it  does  post  mortem. 

(b)  The  insoluble  proteid  of  muscle,  Myostromin,  seems 
to  be  in  the  nature  of  a  nuclein,  and  probably  forms  the 
framework   of   the   fibres.      It   is   always  mixed   with   the 
collagen   of  the   fibrous   tissue   of  muscle,  and  it  may  be 
separated  by  dissolving  in  carbonate  of  soda  solution.     (See 
Chemical  Physiology,  p.  6.) 

Collagen  of  the  fibrous  tissue  holding  the  muscle  fibres 
together  is  also  present,  and  yields  gelatin. 

In  addition  to  the  proteids,  small  quantities  of  other 
organic  substances  are  found  in  muscle. 

2.  Carbohydrates. — Glucose  (C6H1206)  is  present  in  muscle, 
as  in  all  other  tissues. 

Glycogen  #(C5H10O5) — a  substance  closely  allied  to  ordi- 
nary starch,  but  giving  a  brown  reaction  with  iodine — is 
always  present  in  muscle  at  rest.  If  the  muscle  has  been 
active,  the  amount  of  glycogen  diminishes,  being  probably 
converted  to  glucose,  and  used  for  the  nourishment  of  the 
tissue.  (For  the  chemistry  of  these,  see  p.  316.) 


THE   TISSUES  43 

3.  Fat  is  present  in  small  quantities. 

4.  Inosite,  formerly  called  muscle  sugar,  is  present  in  small 
amounts.     It  is  not  a  sugar,  but  a  benzene  compound. 

5.  Sarcolactic  Acid. — Hydroxy-propionic  acid — 

H  OH  0 
H— C— C— C— 0— H 


This  isomer  of  ordinary  lactic  acid  is  increased  in  muscle 
during  activity  and  during  death  stiffening. 

6.  Extractives. — If  dried  muscle  is  treated  with  alcohol  a 
series  of  bodies  containing  nitrogen  may  be  extracted.  The 
chief  of  these  is  Creatin,  or  methyl-guanidin-acetic  acid. 
Guanidin  C.NH(NH2)2  is  a  near  ally  of  urea  CO(NH2)2 
being  formed  by  replacing  the  0  with  NH. 

Methyl-guanidin  is  produced  by  replacing  an  H  in  gua- 
nidin  by  CH3 — 

NH   CH, 

I!       I  ' 
H2N— C  —  N— H 

If  this  is  linked  to  acetic  acid — 


H—  N 


H2N—  C—  N- 
Methyl-guanidin 


H    0 

-LL 


OH 

acetic  acid 


is  produced. 


7.  The  Colour  of  Muscle  varies  considerably,  some  muscles 
being  very  pale,  almost  white  in  colour  —  e.g.  the  breast 
muscles  of  the  fowl ;  others  again  being  distinctly  red,  even 
after  all  the  blood  has  been  removed.  This  red  colour  is,  in 
some  cases,  due  to  the  presence  of  the  pigment  of  blood, 
hcemoglobin,  but  in  certain  muscles  it  is  due  to  a  peculiar 
set  of  pigments,  Myohaematins,  giving  different  reactions  from 
the  blood  pigment. 


44  HUMAN   PHYSIOLOGY 

3.  Physical  Characters  of  Muscle. 

1.  Muscle  is  translucent  during  life,  but,  as  death  stiffen- 
ing sets  in,  it  becomes  more  opaque. 

2.  Muscle  is  markedly  extensile  and  elastic.     A  small  force 
is  sufficient  to  change  its  shape,  but  when  the  distorting  force 
is  removed  it  returns  completely  to  its  original  shape,  pro- 
vided always  that  the  distortion  has  not  overstepped  the 
limits  of  elasticity. 

When  a  distorting  force  is  suddenly  applied  to  muscle — 
e.g.  if  a  weight  is  suddenly  attached — the  distortion  takes 
place  at  first  rapidly,  and  then  more  slowly,  till  the  full  effect 
is  produced.  If  now  the  distorting  force  is  removed  the 
elasticity  of  the  muscle  brings  it  back  to  its  original  form, 
at  first  rapidly,  and  then  more  slowly.  (Practical  Physi- 
ology, Chap.  VI.) 

The  advantages  of  these  properties  of  muscle  are,  that 
every  muscle  in  almost  all  positions  of  the  parts  of  the  body 
is  stretched  between  its  point  of  origin  and  insertion.  When 
it  contracts  it  can  therefore  at  once  act  to  bring  about  the 
desired  movement,  and  no  time  is  lost  in  preliminary  tighten- 
ing. Again,  the  force  of  contraction,  acting  through  such  an 
elastic  medium,  causes  the  movement  to  take  place  more 
smoothly,  and  without  jerks.  Experimentally,  too,  it  has 
been  ascertained  that  a  force  acting  through  such  an  elastic 
medium  produces  more  work  than  when  it  acts  through  a 
rigid  medium. 

The  extensibility  of  muscle  is  of  value  in  allowing  a  group 
of  muscles  to  act  without  being  strongly  opposed  by  their 
antagonistic  group.  For  instance,  suppose  the  extensor 
muscles  of  the  arm  were  not  readily  extensile,  when  the 
flexors  acted,  a  large  amount  of  their  energy  would  have  to 
be  employed  in  elongating  the  extensors.  Similarly  the 
elasticity  of  the  muscles  tends  to  bring  the  parts  back  to 
their  normal  position  when  the  muscles  have  ceased  to 
contract.  It  must  not,  however,  be  imagined  that,  in  all 
movements  of  one  set  of  muscles,  the  antagonistic  muscles 
are  relaxed,  although  they  may  be  elongated.  Often  they 
are  in  a  state  of  activity  so  as  to  guide  the  movements  being 
produced  (see  p.  60). 


THE  TISSUES  45 


Tonus  of  Muscle.  —  The  tense  condition  of  resting 
between  its  points  of  origin  and  insertion  is  not  merely  due 
to  passive  elasticity,  but  is  in  part  caused  by  a  continuous 
contraction,  kepj^up  by  the  action  of  the  nervous  system.     Jf^ 
the  nerve  to  a  group  of  muscles  be  cut,  the  muscles  become 
soft  aridHabby,  and  lose  their  tense  feeling. 

3.  Heat  Production.  —  Muscles,  like  all  other  living  proto- 
plasm, is  in  a  state  of  continued  chemical  change,  constantly 
undergoing  decomposition  and  reconstruction.  •  As  a  result 
of  this  chemical  change,  heat  is  evolved.     This  is  a  matter 
of  considerable  importance,  as  it  explains  how,  even  when  the 
muscles  are  at  rest,  the  body  is  still  kept  at  a  comparatively 
high  temperature. 

4.  Electrical  Conditions.  —  Muscle  when  at  rest  is  iso- 
electric,  but  if  one  part  is  injured,  it  acts  to  the  rest  like  the 
zinc  plate  in  a  galvanic  battery  —  becomes  electro-positive  ;  and 
hence  if  a  wire  passes  from  the  injured  to  the  uninjured  part 
round  a  galvanometer,  a  current  is  found  to  flow  along  the 
wire  from  the  uninjured  to  the  injured  part  just  as,  when  the 
zinc  and  copper  plates  in  a  galvanic  cell  are  connected,  a  cur- 
rent flows  through  the  wire  from  copper  to  zinc.     This  is  the 
Current   of  Injury  (p.  65).       (Practical  Physiology,  Chap. 
VIII.) 

2.  METHODS  OF  MAKING  MUSCLE  CONTRACT. 

Skeletal  muscle  remains  at  rest  indefinitely  until  stimu- 
lated to  contract,  usually  by  changes  in  the  nerves.  We 
desire  to  contract  our  biceps  :  certain  changes  occur  in  our 
brain,  these  set  up  changes  in  the  nerves  passing  to  the 
biceps  and  the  muscle  contracts. 

Can  skeletal  muscle  be  made  to  contract  without  the 
intervention  of  nerves  —  can  it  be  directly  stimulated  ? 

To  answer  this,  some  means  of  throwing  the  nerves  out  of 
action  must  be  had  recourse  to.  If  curare,  a  South  American 
arrow  poison,  be  injected  into  an  animal  —  e.g.  into  a  frog,  the 
brain  of  which  has  been  destroyed  —  it  soon  loses  the  power 
of  moving.  When  the  nerve  to  a  muscle  is  stimulated,  the 
muscle  no  longer'  contracts.  But  if  th^  TmiPfi1ft  ^Q  ^ir^tly 
stimulated  by  any  of  the  various  agents  to  be__afterwards 
m  entidneH,  it  at  once,  contracts.  ~~ 


46  HUMAN   PHYSIOLOGY 

It  might  be  urged  that  the  curare  leaves  unpoisoned  the 
endings  of  the  nerve  in  the  muscle,  and  that  it  is  by  the 
stimulation  of  these  that  the  muscle  is  made  to  contract. 
But  that  these  are  poisoned  is  shown  by  the  fact  that  if  the 
artery  to  the  leg  be  tied  just  as  it  enters  the  muscle,  so  that 
the  poison  acts  upon  the  whole  length  of  the  nerve  except 
the  nerve  endings  in  the  muscle,  stimulation  of  the  nerve 
still  causes  muscular  contraction.  Only  when  the  curare  is 
allowed  to  act  upon  the  muscle  and  the  nerve  endings  in  the 
muscle^ does  stimulation  of  tEe~nerve  fail  to  produce  any  re- 
action in  the  muscle,  while  direct  stimulation  of  the  muscle 

causes  it  to  contract.  This 
clearly  shows  that  it  is  the 
nerve  endings  which  are  poi- 
soned by  _jzygdre,  and  that 
therefore  the  application  of 
stimuli  to  the  muscle  must 

FIG.  16. — Curare  Experiment  to  show  ,    j.        ,1  x,  , 

sciatic  nerves  exposed   to  curare,  ^Ct  directly  Upon  the    muscular 

but  nerve  endings  protected  on  the  fibres       (Fig.      16).         (Practical 

left  side;    while  on  the  right  side  p^flfc™    Chap<  IV<) 
the  curare  is  allowed  to  reach  the  y  &*' 

nerve  endings  in  the  muscle.  Muscle,     however,      is      more 

readily  stimulated  through  its 

nerves,  and  a  knowledge  of  the  points  of  entrance  of  the 
nerves  into  muscles,  the  motor  points,  is  of  importance  in 
medicine  in  indicating  the  best  points  at  which  to  apply 
electrical  stimulation. 

Various  means  may  be  used  to  make  the  muscle  contract. 

1st.  Various  chemical  substances  when  applied  to  a  muscle 
make  it  contract  before  killing  it,  while  others  kill  it  at  once. 
Among  the  former  may  be  mentioned  dilute  mineral  acids 
and  metallic  salts.  (Practical  Physiology,  Chap.  II.) 

2nd.  A  sudden  mechanical  change  such  as  may  be  pro- 
duced by  pinching,  tearing,  or  striking  the  muscle  will  cause 
it  to  contract.  (Practical  Physiology,  Chap.  II.) 

3rd.  Any  sudden  change  of  temperature,  either  heating  or 
cooling,  stimulates  muscle.  A  slow  change  of  temperature 
has  little  or  no  effect.  Every  muscle,  however,  passes  into  a 
state  of  contraction — heat  stiffening  when  a  sufficiently  high 
temperature  to  coagulate  its  proteid  constituents  is  reached. 


THE   TISSUES 


47 


This,  however,  is  not  a  true  living  contraction.  (Practical 
Physiology,  Chap.  III.) 

4th.  Muscle  may  also  be  made  to  contract  by  any  sudden 
change  in  an  electric  current  passed  through  it,  whether  the 
current  be  suddenly  allowed  to  pass  into  it  or  suddenly  cut 
out  of  it,  or  whether  it  is  suddenly  made  stronger  or  weaker. 
(Practical  Physiology,  Chap.  II.) 

This  method  of  stimulating  muscle  is  constantly  used  in 
medicine.  It  is  a  matter  of  no  importance  how  the  elec- 
tricity is  procured,  but  most  usually  it  is  obtained  either — 

1st.  Directly  from  a  galvanic  battery ;  or 

2nd.  From  an  induction  coil. 


FIG.  17. — Rheonome,  consisting  of  a  circular  trough  filled  with  sulphate  of 
zinc  solution,  into  which  dip  the  arras  of  a  bridge  which  can  be  brought 
rapidly  or  slowly  into  the  proximity  of  the  wires  of  a  galvanic  circuit. 

If  a  galvanic  battery  is  used — (1)  On  making  (closing)  the 
current,  and  upon  breaking  (opening)  the  current,  a  contrac- 
tion results.  While  the  current  is  flowing  through  the  muscle, 
the  muscle  usually  remains  at  rest ;  but  if  the  current  is 
suddenly  increased  in  strength  or  suddenly  diminished  in 
strength,  the  muscle  at  once  contracts.  With  strong  cur- 
rents a  sustained  contraction  —  galvanotonus  —  may  persist 
while  the  current  flows.  (Practical  Physiology,  Chap.  II.) 

It  is  the  suddenness  in  the  variation  of  the  strength  of  the 
current  rather  than  its  absolute  strength  which  is  the  factor 
in  stimulating,  as  may  be  shown  by  inserting  some  form  of 
rheonome  into  the  circuit  by  which  the  current  may  be  either 
slowly  or  rapidly  varied  (Fig.  17). 


48  HUMAN  PHYSIOLOGY 

(2)  If  a  current  be  made  weaker  and  weaker,  breaking 
ceases  to  cause  a  contraction,  while  making  still  produces  it. 
That  is,  it  requires  a  stronger  current  to  cause  a  contraction 
on  breaking  than  on  making. 

(3)  The  two  poles  do  not  produce  the  same  effect.     The 
negative  pole  or  cathode — that  coming  from  the  zinc  plate 
of  the  battery — causes  contraction  of  the  muscle  on  closing ; 
while  the  positive  pole  or  anode  causes  contraction  at  opening. 
This  may  be  summarised  as  follows : — 

1.  Contraction  on  closing ;  contraction  on  opening. 

2.  Closing  contraction  stronger  than  opening  contraction. 

3.  Contraction  at  cathode  on  closing,  at  anode  on  opening. 
(Practical  Physiology,  Chap.  VII.) 

1.  C.C  •  C.O 

2.  CC  >  CO 

3.  CCC     CAO 
How  can  these  facts  be  explained  ? 

Electrotonus. 

A  study  of  the  influence  of  the  current  on  the  muscle 
while  it  is  passing  through  it  throws  important  light  on  this 
point.  (Practical  Physiology,  Chap.  VII.) 

While  the  current  simply  flows  through  the  muscle  no 
contraction  is  produced,  but  the  excitability  is  profoundly 
modified. 

Round  the  cathode  it  becomes  more  easily  stimulated, 
while  round  the  anode  or  positive  pole  it  becomes  less 
easily  stimulated.  This  may  be  expressed  by  saying  that 
the  part  of  the  muscle  under  the  influence  of  the  cathode  is 
in  a  state  of  cathelectrotonus,  of  increased  excitability  or  of 
more  unstable  equilibrium,  while  the  part  of  the  muscle 
under  the  influence  of  the  anode  is  in  a  state  of  anelectro- 
tonus,  of  decreased  excitability  or  of  more  stable  equilibrium. 
Now  it  is  well  known  that  any  sudden  disturbance  of  the 
equilibrium  or  balance  of  a  series  of  bodies  is  apt  to  cause 
them  to  fall  asunder.  If,  for  instance,  from  a  house  of  cards, 
one  card  is  suddenly  drawn  out,  the  whole  structure  passes 
into  a  condition  of  unstable  equilibrium,  and  is  apt  to  fall 
to  pieces.  So  with  a  muscle,  if  it  is  suddenly  made  unstable 
as  at  the  cathode  on  closing,  disintegration  or  katabolic 


THE  TISSUES 


49 


changes  are  apt  to  be  set  up  and  a  contraction  results.  If, 
on  the  other  hand,  a  house  of  cards  is  built  and  made  extra 
stable  by  introducing  some  additional  cards  at  the  founda- 
tion, if  these  cards  are  suddenly  withdrawn  the  chances 
are  that  the  house  falls  to  bits.  So  with  a  muscle.  When 
the  current  is  opened  the  removal  of  the  state  of  increased 
stability  at  the  positive  pole  may  cause  disintegration  and 
produce  the  anodal  opening  contraction. 

The  study  of  electrotonus  thus  explains  why  any  sudden 
change  in  the  flow  of  electricity  through  a  muscle  stimulates 
it.  It  further  explains  why  the  stimulation  and  contraction 
start  from  the  cathode  on  closing  and  from  the  anode  on 
opening;  and  why  the  closing  contraction  is  stronger  than 
the  opening,  since  the  sudden  pro- 
duction of  a  condition  of  actual  in- 
stability must  act  more  powerfully 
than  the  simple  sudden  removal  of 
a  condition  of  increased  stability. 

This  law  of  Polar  Excitation,  while 
it  applies  to  muscle  and  nerve,  does 
not  apply  to  all  protoplasm.  Thus 
amoeba  shows  contraction  at  the 
anode  and  expansion  at  the  cathode 
when  a  galvanic  current  is  passed 
through  it. 


When  muscle  is  stimulated  by 
induced  electricity  (Fig.  21)  the  ques- 
tion is  much  easier,  for,  with  each  make 
and  break  or  each  sudden  alteration 
in  the  strength  of  the  primary  circuit, 
there  is  a  sudden  appearance  and 
equally  sudden  disappearance  of  a 
flow  of  electricity  in  the  secondary 
coil.  If,  therefore,  wires  from  the 
secondary  coil  are  led  off  to  a  muscle,  each  change  in  the 
primary  circuit  causes  the  sudden  and  practically  simul- 
taneous appearance  and  disappearance  of  an  electric  current 
in  the  muscle,  and  this  of  course  causes  a  contraction. 
But  here  the  effects  of  closing  and  opening  the  current  are 

4 


FIG.  18.— Course  of  Electric 
Current  in  primary  circuit 
(lower  line),  and  in  secon- 
dary circuit  (upper  line)  of 
an  induction  coil.  Observe 
that  in  the  secondary  the 
make  (upstroke)  and  break 
(downstroke)  are  com- 
bined, and  that  a  stronger 
current  is  developed  in 
the  secondary  circuit  upon 
breaking  than  upon  mak- 
ing the  primary  circuit. 


50  HUMAN  PHYSIOLOGY 

practically  fused,  and  hence  the  influence  of  the  anode  and 
cathode,  or  of  closing  and  opening,  need  not  be  considered. 
(Practical  Physiology,  Chap.  II.) 

It  must,  of  course,  be  remembered,  that  the  opening  of 
the  primary  circuit  produces  a  more  powerful  current  in  the 
secondary  coil,  and  therefore  a  more  powerful  stimulation  of 
the  muscle  than  the  closure  of  the  primary  circuit  (Fig.  18). 

When  the  galvanic  current  is  used  to  stimulate  muscles 
through  the  skin  in  man  or  in  other  living  animals,  the 
different  action  of  the  two  poles  is  not  so  marked  as  in  the 

Cathode  Anode 


Skin 


Muscle 


FlO.  19. — Electrical  Stimulation  of  human  muscle  or  nerve  to  show  the  passage  of 
the  current  across  the  structure,  and  the  consequent  combination  of  effects 
under  each  pole. 

excised  muscle  of  the  frog,  because  the  current,  passing 
through  the  skin  above  the  muscle  to  enter  the  body,  flows 
not  along  but  rather  across  the  muscle,  and  thus,  under  each 
pole  applied  to  the  skin  there  is  on  one  side  of  the  muscle 
the  effect  of  an  entering  current — anode — and  on  the  other, 
of  a  leaving  current— -cathode  (Fig.  19).  Thus  the  same  bit 
of  muscle  or  nerve  is  subjected  to  anelectrotonus  on  one 
side  and  cathelectrotonus  on  the  other,  and  the  effects  of 
closing  and  of  opening  therefore  tend  to  be  combined. 
Hence  with  a  strong  current  contraction  occurs  both  on 
closing  and  on  opening  at  both  poles,  but  as  the  current 
is  weakened  first  the  contraction  at  the  cathode  on  opening, 
next  at  the  anode  on  opening,  then  at  the  anode  on  closing, 


THE   TISSUES  51 

and,  finally,  at  the  cathode  on  closing,  disappear.  When 
the  muscle  is  in  one  stage  of  the  degeneration  which  follows 
separation  from  its  nerve,  the  anodal  closing  contraction 
(Fig.  19,  Anode  M)  becomes  much  exaggerated.  This  is 
called  the  reaction  of  degeneration. 


3.  THE  CHANGES  IN  MUSCLE  DURING  CONTRACTION. 
I.  Change  in  Shape. 

The  most  manifest  change  is  an  alteration  in  the  shape 
of  the  muscle.  It  becomes  shorter  and  thicker.  This 
any  one  can  see  by  studying  their  own  biceps  muscle. 
Contraction  of  muscle,  however,  is  not  a  necessary  result 
of  excitation.  Thus  a  part  of  a 
muscle  when  dipped  in  water  may 
fail  to  contract  when  stimulated,  but 
may  manifest  its  excitation  by  caus- 
ing the  part  of  the  muscle  not  in  the 
water  to  contract — by  conducting. 

In  skeletal  muscle  the  shortening 
and  thickening  of  the  muscle  as  a 
whole  is  due  to  the  shortening  and 
thickening  of  the  individual  fibres 
and  their  fibrils. 

In  these  fibrils  the  shortening  and 
thickening  is  most  marked  in  the 
dim  band.  The  clear  band  also 
shortens,  and  at  the  same  time  it 
becomes  darker  till,  in  the  fully  con- 
tracted muscle,  it  may  be  as  dark  as 
the  dim  band. 

These  appearances  may  best  be  explained  on  the  assump- 
tion that  the  fibrils  are  the  part  of  the  fibre  which  shorten  and 
thicken,  possibly  by  a  flow  of  fluid  into  them,  and  that  these 

fibrils  chiefly  shorten  where  they  are  thickest in  the  dim 

band.  At  the  same  time  by  the  contraction  of  the  fibrils  in 
the  clear  band,  adjacent  dim  bands  may  be  supposed  to  be 
pulled  nearer  to  one  another,  and  to  cast  a  shadow  over  the 
clear  band.  That  no  actual  chemical  change  takes  place  in 


FIG.  20.  —  Contraction  of 
Skeletal  Muscle  —  relaxed 
above,  contracted  below. 
A  is  a  diagram  of  the 
change  in  a  fibril ;  B  shows 
the  shading  of  the  clear 
band ;  and  C  shows  the 
absence  of  any  alteration 
in  the  influence  of  the  two 
bands  on  polarised  light. 


52  HUMAN  PHYSIOLOGY 

the  clear  band  seems  to  be  indicated  by  the  facts  that  it 
retains  its  reactions  to  polarised  light  and  staining  reagents. 

"Usually  the  contraction  of  a  muscle  occurs  simultaneously 
in  all  the  fibres.  This  is  because  a  nerve  fibre  passes  to  every 
muscular  fibre,  and  sets  them  all  in  action  together.  When, 
as  sometimes  occurs  in  disease,  the  nerve  fibres  become 
implicated,  the  muscular  fibres  may  not  all  act  at  once,  and 
a  peculiar  fibrillar  twitching  of  the  muscle  may  be  produced. 
]  If  the  muscle  be  directly  stimulated  at  any  point,  the 
contraction  starts  from  that  point  and  passes  as  a  wave  of 
'contraction  outwards  along  the  fibres.  This  may  be  seen  by 
sharply  percussing  the  fibres  of  the  pectoralis  major  in  the 
chest  of  an  emaciated  individual.  The  rate  at  which  the 
wave  of  contraction  travels  is  ascertained  by  finding  how 
long  it  takes  to  pass  between  any  two  points  at  a  known 
distance  from  one  another.  Its  velocity  is  found  to  vary 
much  according  to  the  kind  of  muscle  and  the  condition  of 
the  muscle.  In  the  striped  muscular  fibres  of  a  frog  in  good 
condition  it  travels  at  something  over  three  metres  per  second. 
When  the  muscle  is  in  bad  condition  the  wave  passes  more 
slowly,  and  in  an  exhausted  muscle  it  may  remain  at  the 
point  of  stimulation.  (Practical  Physiology,  Chap.  IV.) 

The  cause  of  the  propagation  of  this  wave  is  simply  the 
continuity  of  the  muscle  fibres.  The  fibres  stimulated  are 
set  in  action,  and  the  evolution  of  energy  in  these  stimulates 
the  adjacent  fibres,  and  so  the  contraction  passes  along  the 
muscle  as  a  flame  passes  down  a  trail  of  gunpowder. 

Contraction  of  Muscle  as  a  whole  may  best  be  studied 
under  the  following  heads  : — 

1st.  The  course  of  contraction. 
2nd.  The  extent  of  contraction. 
3rd.  The  force  of  contraction. 

1st.  Course  of  Contraction  (Fig.  21). 

By  attaching  the  muscle  (M)  to  a  lever  (L\  and  allowing 
the  point  of  the  lever  to  mark  upon  some  moving  surface, 
a  magnified  record  of  the  shortening  of  the  muscle  when 
stimulated  may  be  obtained. 

A  revolving  cylinder  covered  with  a  smoke-blackened, 
glazed  paper  is  frequently  used  for  this  purpose,  and  to 


THE  TISSUES 


or 


53 


stimulate  and  mark  the  moment  of  stimulation  an  induction 
coil  (p.c.,  s.c.),  with  an  electro-magnetic  marker  (T.M.),  intro- 
duced in  the  primary  circuit,  may  be  employed. 


B 


s.c, 


FIG.  21.— A,  Method  of  Recording  Muscular  Contraction.  B,  Key  to  Parts  of 
Apparatus.  M,  Muscle  attached  to  crank  lever  L.  p.c.,  Primary  circuit, 
and,  s.c.,  secondary  circuit  of  an  induction  coil  with  short  circuiting  key,  k', 
in  secondary  circuit.  B,  Galvanic  cell,  and,  k,  a  mercury  key  for  closing  and 
opening  the  primary  circuit.  T.M.,  A  lever  moved  by  an  electro-magnet 
placed  in  the  primary  circuit  and  marking  the  moment  of  stimulation.  In  As 
a  tuning  fork  beating  100  times  per  second  is  shown  recording  its  vibration 
on  the  drum. 


54 


HUMAN  PHYSIOLOGY 


To  find  the  duration  of  the  contraction,  a  tuning  fork, 
vibrating  100  times  per  second,  may  be  made  to  record  its 
vibrations  on  the  surface.  (Practical  Physiology,  Chap. 

in.) 

In  this  way  such  a  tracing  as  is  shown  in  Fig.  22  is  pro- 
duced. 

From  this  it  is  evident  that  the  muscle]  does  not  contract 
the  very  moment  it  is  stimulated,  but  that  a  short  latent 
period  supervenes  between  the  stimulation  and  the  contrac- 

tion. In  the  muscle  of  the 
frog  attached  to  a  lever  this 
usually  occupies  about  T^th 
second  ;  but  if  the  change  in 
the  muscle  is  directly  photo- 
graphed  without  any  lever 


FIG.  22. — Trace  of  Simple  Muscle 
Twitch  (1)  showing  periods  of 
latency,  contraction,  and  relaxa- 
tion ;  record  of  moment  of  stimu- 
lation (2) ;  and  a  time  record  made 
with  a  tuning  fork  vibrating  100 
times  per  second  (3). 


being  attached  to  it,  this 
period  is  found  to  be  very 
much  shorter. 

The  latent  period  is  fol- 
lowed by  the  period  of  con- 
traction. At  first  it  is  sudden, 
but  it  becomes  slower,  and  finally  stops.  Its  average  dura- 
tion in  the  frog's  muscle  is  about  -j-J^-th  second. 

The  period  of  relaxation  follows  that  of  contraction,  and 
it  depends  essentially  on  the  elasticity  of  the  muscle,  whereby 
it  tends  to  recover  its  shape  when  the  distorting  force  is  re- 
moved. The  recovery  is  therefore  at  first  fast  and  then  slow, 
and  it  lasts  in  the  frog's  muscle  about  To4rth  second. 

The  whole  contraction  thus  lasts  only  about  TVth  second 
in  the  frog's  muscle.     In  mammalian  muscle   it  is  much 
shorter,  and  in  the  muscle  of  insects  shorter  still. 
2nd.  Extent  of  Contraction. 

While,  as  will  be  afterwards  considered,  the  extent  of  con- 
traction is  modified  by  the  strength  of  stimulus  and  the  state 
of  the  muscle,  the  total  amount  of  contraction  is  primarily 
determined  by  the  length  of  the  muscle.  If  a  muscle  of  two 
inches  contracts  to  one-half  its  length,  the  amount  of  con- 
traction is  one  inch,  but  if  a  muscle  of  four  inches  contracts 
to  the  same  amount,  it  shortens  by  two  inches. 

3rd.  Force  of  Contraction  is  measured  by  finding  what 


THE  TISSUES  55 

weight  the  muscle  can  lift,  and  the  absolute  force  of  a  muscle 
may  be  expressed  by  the  weight  which  is  just  too  great  to  be 
lifted.  The  lifting  power  of  a  muscle  depends  primarily  upon 
its  thickness  or  sectional  area.  The  absolute  force  of  a  muscle 
may  therefore  be  expressed  per  unit  of  sectional  area.  In 
man  the  absolute  force  per  1  sq.  cm.  is  from  5000  to  10,000 
grams.  The  force  of  contraction  is,  however,  modified  by  so 
many  other  conditions  that  no  definite  figure  can  be  given. 

The  force  of  contraction  during  different  parts  of  the  con- 
traction period  may  be  recorded  by  making  the  muscle  pull 
upon  a  strong  spring,  so  that  it  can  barely  shorten.  The 
slight  bending  of  the  spring  may  be  magnified  and  recorded 
by  a  long  lever,  and  in  this  way  it  is  found  that  the  ordinary 
curve  of  contraction  gives  a  fair  representation  of  the  varia- 
tions in  the  force. 

This  method  of  recording  the  force  of  contraction  is  some- 
times called  the  isometric  method,  in  distinction  to  the 
isotonic  method  of  letting  the  muscle  act  on  a  light  lever. 
In  clinical  medicine  the  DYNAMOMETER  is  used  for  measuring 
the  force  of  muscular  contraction.  (Practical  Physiology, 
Chap.  V.) 

II.  The  Factors  modifying  the  Contraction. 

1.  Kind  of  Fibre. — In  skeletal  muscles  the  pale  fibres  con- 
tract more  rapidly  and  completely  than  the  red  fibres.     The 
peculiarities  of  the  contraction  of  visceral  muscles  will  be 
considered  later  (p.  66). 

2.  Species  of  Animal. — In  vertebrates  the  contraction  of 
the  muscles  of  warm-blooded  animals  is  more  rapid  than  the 
contraction  in  cold-blooded  animals.     The  most  rapidly  con- 
tracting muscles  are  met  with  in  insects. 

3.  State   of  the   Muscle. — (1)  Continued  Exercise. — If  a 
muscle  is  made  to  contract  repeatedly,  the  contractions  take 
place  more  and  more  slowly.  At  first  each  contraction  is  greater 
in  extent,  but,  as  the  contractions  go  on,  the  extent  diminishes 
as  fatigue  becomes  manifest,  and  finally  stimulation  fails  to 
call  forth  any  response.     This  condition  is  probably  caused 
by  the  accumulation  of  the  products  of  activity  \\\  t.TiP  TY^^IA 
acting  as  poisons  upon  its  protoplasm,  for  the  same  pheno- 


HUMAN  PHYSIOLOGY 


mena  may  be  induced  by  the  application  of  dilute  acids  and 
certain  other  drugs,  and  may  be  removed  for  a  time_by.jyash- 

out    the   muscle  with    salt 


FIG.  23. — Influence  of  continued 
Exercise  on  Skeletal  Muscle— (1) 
the  first  trace ;  (2)  a  trace  after 
moderate  exercise ;  (3)  a  trace 
when  fatigue  has  been  induced. 


solutions^  (Fig.   23).     (Practical 
Physiology,  Chap.  III.) 

(2)  Temperature.  —  If  a  muscle 
be  warmed  above  the  normal 
temperature  of  the  animal  from 
which  it  is  taken,  all  the  phases 
ofcontraction  become  more  rapid, 
and  the~  contraction  is  jit  first 
increased  in  extent,  but  subse- 
quently decreased  in  force.  If, 

on  the  other  hand,  a  jmu_sdfi_hfi_  cooleil,  the  various  periods 
are  prolonged.  At  first  the  contraction  becomes  greater 
and  more  powerful,  but  as  the  cooling  process  goes  on 
it  becomes  less  and  less,  until  finally  the  most  powerful 
stimuli  produce  no  effect.  Cooling  has  thus  practically  the 
same  effect  as  fatigue  (Fig.  23).  (Practical  Physiology,  Chap. 
III.) 

4.  Drugs.  —  Many  drugs  modify  muscular  contraction,  e.g. 
veratrin  enormously  prolongs  the  relaxation  period.    (Prac- 
tical Physiology,  Chap.  IV.) 

5.  Strength  of  Stimulus.  —  A  stimulus  must  have  a  certain 
intensity    to    cause    a 

contraction.  The  pre- 
cise  strength  of  this 
minimum  stimulus 
depends  upon  the  con- 
dition of  the  muscle. 
The  application  of 
stronger  and  stronger 
stimuli  causes  the 
muscular  contraction 
to  become  more  and 
more  rapid,  more  and 
more  complete,  and 


Con. 


Flo.  24. — Influence  of  increasing  the  Strength  of 
the  Stimulus  upon  the  contraction  of  Skeletal 
Muscle.  St.,  the  stimulus;  Con.,  the  result- 
ing contraction.  A ,  a  subminimal  stimulus  ; 
B,  the  minimum  adequate  stimulus  ;  C,  tbe 
optimum  stimulus. 


more  and  more  powerful.     But  increase  in 


the  contraction 
is  notjproportionate  to  the  increase  in  the  stimulus.  If  the 
stimulus  is  steadily  increased,  the  increase  in  contraction 


THE  TISSUES  57 

becomes  less  and  less.  This  may  be  represented  diagram- 
matically  in  the  accompanying  figure,  where  the  continuous 
lines  represent  the  strength  of  the  stimuli,  and  the  dotted 
lines  the  extent  of  the  contractions  (Fig.  24). 

After  a  certain  strength  of  stimulus  has  been  reached, 
further  increase  of  the  stimulus  does  not_cause  any  increase 
in  the  muscular  contraction.  Tljis  smallest  stimulus  which 
causes  the  maximum  muscular  contraction  is  called  the 
optimum  stimulus. 

Increasing  the  str^ngth^nf  t.Vm  pt.imnbia  shortens  the 
latent  periocl/but  lengthens  the  periods  of  contraction  and 
relaxation. 

6.  Resistance  to  Contraction — Weight  to  be  Lifted.— 
Starting  from  the  extent  of  muscular  contraction  without 


IG.  25. — Influence  of  Load  on  a  Muscular  Contraction,  (a)  The  effect  of  increas- 
ing the  load  on  the  extent  of  contraction ;  (b)  the  effect  of  load  on  the  course 
of  contraction. 

any  load,  it  is  found  that  small  weights  attached  to  the 
muscle  actually  increase  the  extent  of  contraction,  but 
greater  weights  diminish  it,  until  finally,  when  a  sufficient 
weight  is^tpplied,  the  muscle  no  longer  contracts  at  all,  but 
may  actually  slightly  lengthen,  because  its  extensibilities 
increased  during  contraction  (Fig.  25,  a). 

The  application  of  weights  to  a  muscle  causes  the  latent 
period  and  period  of  contraction  to  be  delayed,  while  it 
renders  the  period  of  relaxation  more  rapid,  and  an  over- 
extension  may  be  produced  followed  by  a  recovery  resem- 
bling a  small  after-contraction  (Fig.  25,  b).  (Practical 
Physiology,  Chap.  VI.) 

7.  Successive  Stimuli. — So  far  we  have  considered  the 
influence  of  a  single  stimulus  on  the  shape  of  muscle.  But 
in  nearly  every  muscular  action  the  contraction  of  the 
muscles  must  last  much  longer  than  y^-th  of  a  second. 


58  HUMAN  PHYSIOLOGY 

How  is  this  continued  contraction  of  muscles  produced  ? 
To  understand  this  it  is  necessary  to  study  the  influence  of 
a  series  of  stimuli  on  muscle. 

If,  to  a  frog's  muscle  which  takes  TVth  of  a  second  to 
contract  and  relax,  stimuli  at  the  rate  of  five  per  second 
are  applied,  it  is  found  that  a  series  of  simple  contractions, 
each  with  an  interval  of  TVth  of  a  second  between  them 
are  produced  (Fig.  26,  1).  If  the  stimuli  follow  one  another 
a  b  c  at  the  rate  of  ten  per  second,  a 

series  of  simple  contractions  is  still 
produced,  but  now  with  no  interval 
between  them. 

r     If  stimuli  be  sent  more  rapidly 
to    the    muscle,   say    at    the    rate 
of  twelve  per  second,  the   second 
stimulus   will    cause   a   contraction 
before  the   contraction  due  to  the 
irst   stimulus   has   entirely   passed 
>ff  (Fig.  26,  2).     The   second  con- 
raction  will  thus  be  superimposed 
>n  the  first,  and  it  is   found   that 
he    second    contraction    is    more 
jomplete    than   the   first,  and    the 
hird  than  the  second.     But  while 


\ 


A 


Fio.  26.—  Effect  of  a  series  of 
Stimuli  on  Skeletal  Muscle. 
(See  text.) 


the  second  contraction  is 


markedly  greater  than  the  first,  the 


third  is  not  so  markedly  greater  than  the  second,  and  each 
succeeding  stimulus  causes  a  less  and  less  increase  in  the 
degree  of  contraction  until,  after  a  certain  number,  no 
further  increase  takes  place,  and  the  degree  of  contraction 
I  is  simply  maintained. 

When  the  contractions  follow  one  another  at  such  a  rate 
that  the  relaxation  period  of  the  first  contraction  has  begun, 
but  is  not  completed,  before  the  second  contraction  takes 
place,  a  lever  attached  to  the  muscle,  and  made  to  write  on 
a  moving  surface,  produces  a  toothed  line.  The  contraction 
is  not  uniform,  but  is  made  up  of  alternate  shortenings  and 
lengthenings  of  the  muscle.  This  constitutes  "incomplete 


If  the  second  stimulus  follows  the  first  so  rapidly  that  the 
contraction  period  has  not  given  place  to  relaxation,  then 


THE  TISSUES  59 

the  second  contraction  will  be  superimposed  on  the  first, 
the  third  on  the  second,  and  so  on  continuously  and 
smoothly  without  any  slight  relaxations,  and  thus  the  lever 
will  describe  a  smooth  line,  rising  at  first  rapidly,  then  more 
slowly,  till  a  maximum  is  reached,  and  being  maintained 
at  this  till  the  series  of  stimuli  causing  the  contraction  is 
removed,  or  until  fatigue  causes  relaxation  of  the  muscle. 
This  is  the  condition  Q{  "  complete  tetanus"  (Fig.  26,  3). 
(Practical  Physiology,  Chap.  V.) 

The  rate  at  which  stimuli  must  follow  one  another 
in  order  to  produce  a  tetanus  depends  on  a  large  number 
of  factors.  Anything  which  increases  the  duration  of  a 
single  contraction  renders  a  smaller  number  of  stimuli  per 
second  sufficient  to  produce  a  tetanus,  and  thus  all  the 
various  factors  modifying  a  single  muscular  contraction, 
modify  the  number  of  stimuli  necessary  to  produce  a  tetanus 
(see  p.  55).  D'Arsonval  has  shown  that  an  alternating 
current  with  very  frequent  interruptions  of  about  1,000,000 
per  second  causes  no  contraction. 

Every  voluntary  contraction  of  any  group  of  the  muscles 
is  probably  of  the  nature  of  a  tetanus  ;  and  the  question  thus 
arises,  at  what  rate  do  the  stimuli  which  cause  such  a  tetanus 
pass  from  the  spinal  cord  to  the  muscles  ? 

In  a  tracing  of  a  continued  voluntary  contraction,  indica- 
tions of  about  ten  variations  per  second  are  to  be  seen, 
while  the  rate  of  the  clonic  tremor  of  the  leg  which  may 
be  produced  during  fatigue  by  supporting  the  weight  of 
the  leg  on  the  toes  is  about  ten,  backward  and  forward 
movement,  per  second,  and  in  various  morbid  muscular 
spasms  the  rate  is  about  the  same.  (Practical  Physiology, 
Chap.  V.) 

All  this  would  seem  to  indicate  that  the  number  of 
stimuli  which  pass  to  human  muscle  from  the  central 
nervous  system  is  probably  about  ten  per  second. 

It  has,  however,  been  found  that  passing  a  strong 
galvanic  current  into  a  muscle  may  lead  to  rhythmic 
contraction,  and  hence  it  is  possible  that  the  contractions 
of  muscle  induced  by  the  central  nervous  system  may 
be  caused  by  a  continued  discharge  from  the  nerve 
centres. 


60  HUMAN  PHYSIOLOGY 


III.  Mode  of  Action  of  Muscles. 

The  skeletal  muscles  act  to  produce  movements  of  th 
body  from  place  to  place,  or  movements  of  one  part  of  th 
body  on  another.  This  they  do  by  pulling  on  the  bony  frame 
work  to  cause  definite  movements  of  the  various  joints. 

In  relationship  to  each  joint  the  muscles  are  arrangec 
in  opposing  sets — one  causing  movement  in  one  direction 
another  in  the  opposite  direction — and  named  according  t< 
their  mode  of  action,  flexors,  extensors,  adductors,  abductors, 
&c.  But  in  the  production  of  any  particular  movement — 
say  flexion  of  the  forearm  at  the  elbow — not  only  are  the 
muscles  manifestly  causing  the  movement  in  contraction, 
but  the  opposing  group,  the  extensors,  are  also  in  action  to 
guide  and  direct  the  force  and  extent  of  the  movement. 
This  Co-operative  Antagonism  of  groups  of  muscles  is  of 
very  great  importance,  since  it  explains  many  of  the  results 
observed  in  paralysis.  Thus,  if  the  extensors  of  the  hand 
be  paralysed,  as  in  lead  palsy,  it  is  found  impossible  to 
clench  the  hand,  although  the  flexors  are  intact.  Again, 
if  part  of  the  brain  which  causes  flexion  of  the  hand  of 
the  monkey  be  stimulated,  and  the  nerve  to  the  flexors 
divided,  the  co-operative  action  of  the  extensors  brings  about 
an  extension  of  the  hand. 

The  muscles  round  the  various  joints  act  on  the  bones, 
arranged  as  a  series  of  levers,  of  the  three  classes  (Fig.  27). 


FlQ.  27. — The  three  types  of  lever  illustrated  by  the  movements 
at  the  ankle  joint. 

1st  Class. — Fulcrum  between  power  and  weight.  In  the 
ankle  this  is  seen  when,  by  a  contraction  of  the  gastro- 
cnemius,  we  push  upon  some  object  with  the  toes. 

2nd  Class. — Weight  between  fulcrum  and  power.  In 
rising  on  the  toes  the  base  of  the  metatarsals  is  the  fulcrum, 


THE   TISSUES  61 

the  weight  comes  at  the  ankle,  and  the  power  on  the 
os  calcis. 

3rd  Class.  —  Power  between  fulcrum  and  weight.  In 
raising  a  weight  placed  on  the  dorsal  aspect  of  the  toes  by 
the  contraction  of  the  extensors  of  the  foot,  we  have  the 
weight  at  the  toes,  the  power  at  the  tarsus,  and  the  fulcrum 
at  the  ankle. 

In  the  other  joints,  actions  involving  the  principle  of  each 
of  these  levers  may  be  found. 

IY.  Muscle  Work. 

As  a  result  of  the  changes  in  shape,  muscle  performs  its 
great  function  of  doing  mechanical  work;  and  the  most 
important  question  which  has  to  be  considered  in  regard 
to  muscle,  as  in  regard  to  other  machines,  is  the  amount 
of  work  it  can  do. 

Since  the  work  done  depends  upon  the  weight  moved  and 
the  distance  through  which  it  is  moved,  the  work-doing 
power  of  muscle  is  governed  by  the 
force  of  contraction,  which  determines 
the  weight  which  can  be  lifted,  and 
by  the  amount  to  which  the  muscle 
can  shorten,  for  this  will  govern  the 
distance  through  which  the  weight 
may  be  moved. 

It  has  been  already  shown  that  the 
force  of  contraction  depends  chiefly  FlQ-  28.— influence  of  the 
upon  the  sectional  area  of  a  muscle.  ^/^ daon^uscle  upon 
A  thick  muscle  is  stronger  than  a 

thinner  one.  But  on  the  other  hand,  the  amount  of  con- 
traction depends  upon  the  length  of  the  muscle,  since  each 
muscle  can  contract  only  to  a  fixed  proportion  of  its  original 
length.  A  glance  at  the  diagram  will  at  once  make  this 
plain  (Fig.  28). 

The  size  of  the  muscle  is  thus  the  first  great  factor  which 
governs  its  work-doing  power.  But  the  many  factors  in- 
fluencing the  force  of  muscular  contraction  also  influence 
the  work-doing  power  of  the  muscle  (see  p.  55).  (Practical 
Physiology,  Chap.  VI.) 


Load  in 
Grams. 

Space  through  which 
lifted  in  mm. 

0 

X 

4-5 

20 

X 

3-0 

40 

X 

2-37 

60 

X 

2-00 

80 

X 

1-75 

100 

X 

1-2 

62  HUMAN   PHYSIOLOGY 

One  factor  requires  special  consideration,  namely,  the 
Load. 

We  have  already  seen  that  as  the  load  is  increased  the 
extent  of  contraction  is  diminished. 

The  following  experiment  will  illustrate  the  influence  of 
increasing  the  load  on  the  work-doing  power  of  a  muscle : — 

Work  in  Gram, 
mm. 

O'O 

60-0 
94-8 
120-0 
140-0 
120-0 

It  will  be  seen  that  increasing  the  load  at  first  increases 
the  amount  of  work  done,  jut  after  a  certain  weight  is 
reacrjecr.  diminishes  it.  There~is,  therefore,  for^very  muscle, 
so  far  as  its  working  power  is  concerned,  an  "  optimum " 
load. 

In  studying  the  amount  of  work  a  muscle  or  set  of 
muscles  can  do,  the  element  of  time  must  always  be  con- 
sidered. Obviously  contracting  muscles  will  do  more  work 
in  an  hour  than  in  a  minute.  Hence,  in  trying  to  form 
any  idea  of  the  amount  of  work  a  muscle  can  do,  this  must 
be  expressed  in  work  units,  per  unit  of  bulk  and  per  unit 
of  time. 

The  average  working  capacity  of  skeletal  muscle  may  be 
estimated  as  follows : — A  labourer  who  raises  130,000  kilos, 
through  one  metre  during  his  eight  hours  of  work  does  a 
good  average  day's  work.  His  muscles  weigh  about  25 
kilos.,  and  thus  each  gram,  of  his  muscle  will  do  5  kilogram- 
metres  per  diem,  or  0*06  gram,  metres  per  second. 

When  required,  much  larger  amounts  of  work  can  be 
done  for  short  periods.  It  has  been  calculated  that  in 
the  sprint  of  a  100  yards  race,  work  is  done  at  something 
like  2  gram,  metres  per  second,  about  thirty  times  the  rate 
at  which  a  labourer's  muscles  work.  But  to  increase  the 
rate  at  which  work  is  done  requires  an  increase  in  the 
expenditure  of  the  energy-yielding  materials  in  greater  pro- 


THE  TISSUES  63 

portion  than  the  increased  work — just  as  to  increase  the 
speed  of  a  ship  or  an  engine  requires  an  increase  of  coal 
consumption  in  a  proportion  roughly  corresponding  to  the 
square  of  the  increased  speed. 


Y.  Heat  Production. 

In  muscle,  as  in  other  machines,  by  no  means  the  whole 
of  the  energy  rendered  kinetic  is  used  for  the  production 
of  mechanical  work.  In  a  steam-engine  much  of  the 
energy  is  dissipated  as  heat,  and  the  same  loss  occurs  in 
muscle. 

If  heat  is  given  off  when  a  muscle  contracts,  either  the 
muscle  itself,  or  the  blood  coming  from  it,  will  become 
warmer.  Hence  to  detect  such  a  change  some  delicate 
method  of  measuring  changes  of  temperature  must  be 
employed.  The  mercurial  thermometer  is  hardly  sufficiently 
sensitive,  and,  therefore,  the  thermo-electrical  method  is  most 
generally  employed.  Various  forms  of  thermopile  may  be 
used. 

The  rise  of  temperature  in  a  muscle  after  a  single  con- 
traction is  extremely  small,  but  after  a  tetanic  contraction, 
lasting  for  two  or  three  minutes,  it  is  very  much  greater. 

The  amount  of  heat  produced  may  be  calculated  if 
(a)  the  weight  of  the  muscle;  (6)  its  temperature  before 
and  after  contraction;  and  (c)  the  specific  heat  of  muscle, 
are  known. 

The  specific  heat  of  muscle  is  slightly  greater  than  that 
of  water,  but  the  difference  is  so  slight  that  it  may 
be  disregarded.  If,  then,  ten  grammes  of  muscle  had  a 
temperature  of  15°  C.  before  it  was  made  to  contract,  and 
a  temperature  of  15*05°  C.  after  a  period  of  contraction,  then 
0*5  gramme-degrees  of  heat  have  been  produced ;  i.e.  heat 
sufficient  to  raise  the  temperature  of  0*5  gramme  of  water 
through  1°  C. 

The  amount  of  heat  produced  by  muscle  in  different 
conditions  varies  so  greatly  that  it  is  unnecessary  to  consider 
it  further. 

Relationship  of  Heat  Production  to  Work  Production. — 
Since  it  is  possible  to  measure  both  the  mechanical  work 


64  HUMAN   PHYSIOLOGY 

done  by  a  muscle  and  the  amount  of  energy  dissipated  as 
heat,  it  is  possible  to  determine  the  relationship  of  these 
to  one  another,  and  thus  to  compare  muscle  with  other 
machines  as  to  proportion  of  energy  which  is  utilised  to 
produce  work.  To  do  this  it  is  necessary  to  be  able  to 
convert  "work  units"  into  "heat  units,"  or  vice  versa.  It 
has  been  found  that  0-45  gramme-degrees  is  equivalent  to 
1  kilogram-metre. 

I   The  proportion  of  work  to  heat  is  not  constant.    Gradually 
increasing  the  stimulus  increases  both  work  production  and 
heat  production,  but  the  latter  is  increased  more  rapidly, 
and  reaches  its  maximum  sooner.    Again,  as  muscle  becomes 
f  exhausted  its  heat  production  declines  more  rapidly  than  its 
\work  production.     Exhausted  muscle,  therefore,  works  more 
economically.     If  an  unloaded  muscle  is  made  to  contract  no 
/work  is  done  and  all  the  energy  is  given  off  as  heat,  and  the 
J  same  thing  happens  where  a  muscle  is  so  loaded  that  it 
cannot  contract  when  stimulated. 

But  the  point  of  practical  importance  to  determine  is — 
How  much  of  the  energy  liberated  by  muscle  in  normal 
conditions  is  usually  used  for  mechanical  work,  and  how 
much  is  lost  as  heat?  It  will  afterwards  (p.  312)  be  shown 
that  all  the  energy  of  the  body  comes  from  the  food,  and 
the  amount  of  energy  yielded  by  any  food  may  be  deter- 
mined by  burning  it  in  a  calorimeter.  To  determine  the 
energy  used  in  mechanical  work  some  form  of  work  measurer 
or  ergograph  may  be  used — e.g.  a  wheel  turned  against  a 
measured  resistance.  By  converting  the  work  units  of  the 
work  thus  done  into  heat  units,  and  subtracting  this  from 
the  total  energy  of  the  food,  the  energy  lost  as  heat  may 
be  determined,  and  thus  the  relationship  between  work  pro- 
duction and  heat  production  may  be  found.  By  experiments 
on  men,  horses,  and  dogs,  Zuntz  has  found  that  about  Jrd 
of  the  energy  liberated  may,  under  favourable  conditions,  be 
available  for  mechanical  work,  while  §rds  is  lost  as  heat. 
Compared  with  other  machines,  such  as  steam-engines, 
muscle  must  be  regarded  as  an  economical  worker,  and  it 
has  the  advantage  that  the  heat  liberated  is  necessary  to 
maintain  the  temperature  at  which  the  chemical  changes 
which  are  the  basis  of  life  can  go  on. 


THE  TISSUES  65 


YI.  Electrical  Changes. 

When  a  muscle  contracts  certain  electrical  changes  occur. 
These  may  be  best  studied  in  the  heart,  which  is  a  muscle 
which  can  be  exposed  without  injury.  With  other  muscles 
the  injury  inflicted 
in  isolating  them  sets 
up  electrical  currents 
of  injury  (p.  45). 

If  one  end  of  a  wire       (f  •/  \\ 

be  brought  in  con- 
tact with  the  base  of 
the  ventricle  by  means 
of  a  non-polarisable 

electrode  (in  which  FIG.  29.— To  show  electric  current  of  action  in  a 
SOme  material  which  muscle  (a)  compared  with  that  in  a  galvanic 

J  *-"Urt  cel1   (b)'     Tne   contracting   part  of  the   muscle 

does  not  act  upon  the        is  sh^ded    (g]  Galvanometer. 
muscle    and    is    not 

acted  upon  by  the  muscle  is  in  contact  with  it),  and  another 
wire  be  similarly  connected  with  the  apex,  and  if  these 
wires  are  led  off  round  a  galvanometer,  it  will  be  found 
that  with  each  contraction  of  the  heart  an  electric  current 
is  set  up,  the  one  part  of  the  heart  becoming  first  positive 
and  then  negative  to  the  other  part. 

This  means  that,  when  the  contraction  occurs,  the  part 
which  first  contracts  becomes  of  a  higher  electric  potential 
than  the  rest  of  the  muscle,  so  that  electricity  flows  from 
it  to  the  uncontracted  part  in  the  organ,  and  from  the 
uncontracting  part  to  the  contracting  part  in  the  wire  round 
the  galvanometer.  The  contracting  part  is  thus  similar  to 
the  positive  element  of  a  battery,  the  zinc,  the  uncontracting 
part  to  the  negative  element,  and  the  wire  coming  from  the 
contracting  part  will,  therefore,  correspond  to  the  negative 
pole — that  from  the  uncontracting  part  to  the  positive  pole. 

It  has  now  been  shown  that  this  current  of  action  occurs 
along  with,  and  does  not  precede,  the  period  of  contraction. 
The  electric  change  in  contracting  muscle  may  be  demon- 
strated by  laying  the  nerve  of  a  muscle  nerve  preparation 
over  the  muscle  of  another  muscle  nerve  preparation  or  over 

5 


66  HUMAN  PHYSIOLOGY 

the  beating  heart.  The  current  of  action  in  the  second 
muscle  in  each  case  may  stimulate  the  first.  (Practical 
Physiology,  Chap.  VIII.) 

YII.  Extensibility. 

The  extensibility  of  muscle  is  increased  during  contraction 
so  that  the  application  of  a  weight  causes  a  greater  lengthen- 
ing than  when  the  muscle  is  at  test. 

Visceral  Muscles. 

In  several  important  respects  the  visceral  muscles  differ  in 
their  mode  of  action  from  the  skeletal  muscles. 

1.  Their  connection  with  nerves  is  by  no  means  so  definite 
and   precise,  for,  instead  of  each   nerve-fibre   ending  in   a 
muscle- fibre,   the  nerves   to   non- striped  muscle   form   an 
irregular  network  upon  them,  and  the  muscle-fibre  appears 
to   be   capable   of    action,   both   before   these  nerves   have 
developed  in  the  embryo  and  when  the  influence  of  these 
nerves  has  been  cut  off  in  the  adult.     In  the  intestine  the 
mode  of  action  of  the  muscles  is  largely  dominated  by  the 
plexus  of  nerves  (see  p.  328). 

2.  The  great  features  of  the  action  of  visceral  muscle  are 
— 1st,  its  tendency  to  sustained  tonic  contraction;  and  2nd, 
its  spontaneous  regular  rhythmic  contraction  and  relaxation. 

1st.  The  continuous  slight  tonic  contraction  is  seen  in  all 
the  visceral  muscles,  and,  while  it  may  be  increased  or 
diminished  by  the  intervention  of  nerves,  it  appears  to  be 
largely  independent  and  an  expression  of  the  continuous 
metabolism  of  the  muscle  protoplasm. 

2raZ.  The  rhythmic  contractions  and  relaxations  are  not 
equally  manifest  in  all  situations,  nor  are  they  so  continuous, 
but  they  are  well  marked  in  the  muscles  around  such  hollow 
viscera  as  the  intestines,  bladder,  and  uterus.  Like  the  tonic 
contractions,  they  are  to  a  certain  extent  independent  of 
nerve  action,  but  are  influenced  by  it. 

These  contractions  recur  at  regular  intervals  of  varying 
duration.  Each  contraction  lasts  for  a  considerable  period— 
sometimes  over  a  minute — and  the  relaxation  is  correspond- 
ingly long.  Everything  which  increases  the  rate  of  chemical 


THE  TISSUES  67 

change  increases  the  rapidity  of  the  rhythm.  Thus  warming 
the  muscle  and  the  action  of  a  galvanic  current  have  this 
action. 

3.  When  the  muscle  is  at  rest,  a  contraction  may  be  pro- 
duced by  any  of  the  modes  of  stimulation  which  will  cause 
the  skeletal  muscles  to  contract ;  and  it  may  thus  be  demon- 
strated that  the  latent  period  is  very  long. 

4.  Unlike  skeletal  muscles,  the  extent  of  contraction  is 
not  increased  by  increasing  the  strength  of  the  stimulus. 
The  smallest  available  stimulus  causes  the  maximum  con- 
traction;  but  if  the  same  stimulus  is  repeated  at  regular 
intervals   the    resulting    contractions    become    greater   and 
greater  during  the  application  of  the  first  four  or  five  stimuli, 
so  that  the  record  of  a  series  of  contractions  has  a  somewhat 
stair-like  appearance. 

5.  A  series  of  stimuli  do  not  cause  a  tetanus,  but  simply 
increase  the  rapidity  and  force  of  the  individual  contractions. 

When  the  muscles  are  arranged  round  hollow  viscera,  the 
rhythmic  contraction,  starting  at  one  end,  travels  along  the 
group  of  muscle-fibres  as  a  regular  wave — the  peristaltic 
wave — and  thus  drives  onwards  the  contents  of  the  tube. 
The  rate  at  which  this  wave  travels  varies  very  considerably. 
(Practical  Physiology,  Chap.  IX.) 

Cardiac  Muscle  physiologically  resembles  other  visceral 
muscles,  but  its  period  of  contraction  is  shorter  and  its 
rhythm  generally  more  rapid. 

4.  THE  CHEMICAL  CHANGES  IN  MUSCLE  AND  THE 

SOUKCE   OF   THE   ENERGY   EVOLVED. 

Chemical  changes  are  constantly  going  on  in  muscle,  and 
the  study  of  these  chemical  changes  in  resting  muscle  and 
in  contracting  muscle  explains  the  source  of  the  energy  of 
muscle,  disintegration  leading  to  the  liberation  of  energy  and 
construction  leading  to  the  repair  of  the  muscles  and  the 
storage  of  energy. 

No  part  of  physiology  is  of  more  importance ;  for  it  is  the 
chemical  changes  in  muscle  which  give  rise  to  the  great 
waste  products  of  the  body,  and  it  is  to  make  good  these 
losses  that  fresh  nourishment  has  to  be  supplied.  The 


68  HUMAN  PHYSIOLOGY 

chemical  changes  in  muscle  therefore  govern  both  the  intake 
and  output  of  matter  from  the  body. 

By  studying  the  question  from  a  number  of  different 
standpoints,  and  by  comparing  the  results  so  obtained,  a 
fairly  clear  conception  of  the  chemical  changes  and  the 
source  of  muscular  energy  has  been  obtained. 

1.  Composition  of  Muscle  before  and  after  Contraction. — 
The  method  which  most  naturally  presents  itself  is  to  take 
two  muscles  or  groups  of  muscles  corresponding  to  one 
another,  and  to  examine  the  chemistry  of  one  before  it  had 
been  made  to  contract,  and  of  the  other  after  it  had  been 
contracting  for  some  time. 

Resting  muscle  is  alkaline ;  but  if  an  excised  muscle,  out- 
side the  body,  be  kept  contracting  for  some  time,  it  becomes 
acid,  and  this  acidity  is  due  to  the  appearance  of  sarcolactic 
acid.  Muscle  in  the  body  does  not  become  acid,  because 
the  alkaline  lymph  at  once  neutralises  the  acid  which  is 
produced. 

Again,  after  contraction,  the  glycogen  of  the  muscle  is 
found  to  be  diminished.  But  the  most  important  change  is 
that  the  amount  of  carbon  dioxide,  CO2,  which  can  be  ex- 
tracted from,  muscle  is  very  greatly  increased. 

As  yet  the  changes,  if  any,  in  the  proteids  of  muscle 
during  contraction  have  not  been  fully  investigated,  while 
the  results  of  the  work  accomplished  on  the  nitrogenous 
extractives,  which  are  formed  by  the  decomposition  of  the 
proteids,  are  not  trustworthy.  They  seem  to  indicate  that 
these  bodies  are  increased  during  muscular  contraction  in 
the  excised  muscle.  These  changes  in  a  piece  of  muscle 
may  be  diagrammatically  represented  as  follows  :— 


-f  Carbon  dioxide. 

+  Sarcolactic  acid. 

+  Nitrogenous  extractives  ? 

—  Glycogen. 


The  results  obtained  by  this  method  of  investigation  are 
thus  of  considerable  value,  but  alone  they  give  us  no  clear 
idea  of  the  nature  of  the  chemical  changes. 


THE  TISSUES 


69 


2.  Respiration  of  Excised  Muscle. — By  enclosing  the  ex- 
cised muscle  in  a  closed  space  containing  air  of  known  com- 
position, and  by  investigating  the  changes  in  the  components 
of  the  air  after  the  muscle  has  either  been  kept  at  rest  for 
some  time  or  made  to  contract,  important  light  has  also  been 
thrown  on  these  chemical  changes. 

It  has  been  found  that  the  resting  muscle  constantly  takes 
up  oxygen  from  the  air  round  about  it,  and  constantly  gives 
off  carbon  dioxide.  In  contracting,  more  carbon  dioxide  is 
given  off,  and  usually  the  amount  of  oxygen  taken  up  is  also 
increased.  (Fig.  30.) 

Here  we  have  at  once  evidence  that  muscle  breathes,  and 
that  this  process  of  respiration  is  increased  during  muscular 


FIG.  30.  —  Respiration  of  muscle  in  a  closed  chamber. 

activity.     The  affinity  of  muscle  for  oxygen  is  very  great,  so 
great  that  jt  can  actually  talrp  nvrrpn  n^t  ^f  plifiinjjfiT  com- 


binaiions.  If  methyl  ene  blue  or  alizarin  blue  are  injected 
into  the  vein  of  an  animal,  the  blood  becomes  blue,  but  the 
muscles  remain  colourless,  having  deoxidised  the  pigment 
and  reduced  it  to  a  colourless  condition.  When  freely  ex- 
posed to  air  after  death,  the  blue  colour  returns. 

3.  Changes  in  Blood  passing  through  Muscle.  —  Observa- 
tions on  excised  muscle  carry  us  no  further,  but  by  ascertain- 
ing the  changes  in  composition  which  the  blood  undergoes 
in  passing  through  a  muscle,  further  information  may  be 
gained. 

For  this  purpose  the  two  hind  limbs  of  a  dog  have  been 
used.  The  blood  going  to  one  leg,  and  the  blood  coming 
from  the  other,  are  collected  at  the  same  time.  It  is  found 


70  HUMAN  PHYSIOLOGY 

that  the  blood  in  passing  through  the  muscles  has  gained 
carbon  dioxide  and  lost  oxygen.  If  the  muscles  be  kept 
contracted,  it  is  further  ascertained  that  the  amount  of 
carbon  dioxide  gained  is  increased,  while  usually  the  amount 
of  oxygen  taken  up  is  also  increased.  This  observation  thus 
confirms  the  investigations  on  the  changes  in  the  air  sur- 
rounding a  muscle. 

But  the  solid  constituents  of  the  blood  are  also  changed. 
If  the  muscles  have  been  contracting,  the  blood  is  found  to 
contain  sarcolactic  acid  probably  combined  with  ammonia. 

We  shall  afterwards  find  that  blood  contains  small 
quantities  of  glucose,  C6H1206.  As  it  passes  through 
muscle  it  loses  some  of  this,  even  when  the  muscle  is  at 


,GLUC05  E 
FAT 
OXYGEN 


SARCOLACTIC    ACID 
CARBON      DIOXIDE 


FIG.  31. — Exchanges  between  muscle  and  blood. 

rest,  and  a  much  larger  amount  when  the  muscle  has  been 
active. 

The  changes  in  the  proteids  of  the  blood  going  to  and 
coming  from  muscle  have  not  been  properly  investigated. 

Some  observers  have  obtained  results  which  seem  to 
indicate  that  the  amount  of  fat  which  is  found  in  the  blood 
is  diminished  as  the  blood  passes  through  the  muscles,  but 
whether  this  diminution  is  greater  during  muscular  activity 
has  not  been  studied. 

Such  direct  observations  on  muscle  and  the  blood  nourish- 
ing it  indicate  that  constant  chemical  changes  are  going  on 
when  the  muscle  is  at  rest.  It  is  constantly  giving  off  carbon 
dioxide  and  constantly  consuming  oxygen,  glucose,  and  pos- 
sibly fats  and  proteids.  When  doing  work  these  chemical 
changes  become  more  active.  We  may  compare  resting 


THE  TISSUES  71 

inuscle  as  to  its  chemical  changes  to  an  engine  with  its  fires 
banked  down.  Active  muscle  is  comparable  to  the  engine 
with  its  fires  in  full  blast. 

4.  Effects  of  Muscular  Work  upon  the  Excreta. — Another 
method  of  study  has  yielded  results  of  very  great  value — 
the  investigation  of  the  effects  of  muscular  work  upon  the 
excreta. 

Not  only  is  muscle  the  most  bulky  and  most  constantly 
active  tissue,  but  it  is  the  tissue  in  which  very  extensive 
chemical  changes  occur,  in  the  liberation  of  the  energy  for 
work  and  heat  production,  and  hence  the  waste  products  of 
the  body  are  chiefly  derived  from  muscle,  and  their  amount 
and  character  must  afford  an  indication  of  the  changes  in 
this  tissue. 

This  was  long  ago  recognised,  but  the  older  experimenters 
did  not  sufficiently  realise  that  the  excretions  are  modified 
by  the  amount  and  character  of  the  food  taken,  and  hence 
their  results  are  of  little  value.  In  studying  the  influence 
of  muscular  work  on  the  excreta,  food  must  be  withheld  or 
must  be  unvarying  during  the  experiment. 

If  this  precaution  is  taken,  it  is  found  that  the  excretion 
of  the  various  elements  composing  muscle  is  modified  by 
muscular  work. 

Attention  has  chiefly  been  directed  to  the  variations  in  the 
carbon  and  nitrogen  thrown  off,  the  former  mainly  as  carbon 
dioxide  in  the  expired  air,  the  latter  as  urea  in  the  urine. 
It  has  been  found  that  if  a  fasting  or  underfed  animal  is 
made  to  do  work,  the  excretion  of  both  these  elements  is 
increased,  the  carbon  proportionately  to  the  work  done, 
the  nitrogen  in  quantities  not  strictly  proportionate  to 
the  work,  being  greater  the  more  underfed  the  animal 
is  and  the  harder  the  work  done,  and  being  less  the  better 
nourished  the  animal  is  or  the  less  the  work  that  is  done. 
(Fig.  32,  1). 

If  a  lean  animal  be  fed  on  an  exclusively  proteid  diet,  the 
excretion  of  carbon  and  nitrogen  is  increased,  practically 
proportionately  to  the  work  done  (Fig.  32,  2). 

But  if  the  animal  be  well  fed  on  an  ordinary  diet,  contain- 
ing proteids,  carbohydrates,  and  fats,  the  performance  of 
muscular  work  increases  the  excretion  of  carbon  proportion- 


72  HUMAN  PHYSIOLOGY 

ately  to  the  work  done,  but  may  cause  only  a  very  slight 
increase  in  the  excretion  of  nitrogen  (Fig.  32,  3). 

From  the  increased  excretion  of  nitrogen  and  carbon  the 
consumption  of  proteids  may  be  calculated,  since  proteids 
contain  16  per  cent,  of  nitrogen  and  52  per  cent,  of  carbon — 
i.e.  3*4  times  more  carbon  than  nitrogen.  Each  gram,  of 
nitrogen  excreted  thus  represents  the  breaking  down  of  6-25 
grams,  of  proteid,  and  it  is  accompanied  by  3*4  grams,  of 
carbon.  If  more  carbon  is  excreted,  it  must  come  from 
carbohydrates  or  fat. 

Proceeding  in  this  way,  it  is  found  that  in  the  fasting 
animal  and  in  the  animal  fed  on  proteids,  the  muscles  get 
their  energy  from  proteids,  but  that  in  an  animal  on  an 


.- 

/ 

// 


FIG.  32.— To  illustrate  the  influence  of  Muscular  Work  upon  the 
Excretion  of  Carbon  and  of  Nitrogen — (1)  in  a  fasting  or 
underfed  animal ;  (2)  in  an  animal  fed  on  proteids  ;  (3)  in  an 
animal  on  a  normal  diet. 

ordinary  diet  the  muscles  get  it  chiefly  from  the  carbo- 
hydrates and  fats  of  the  food. 

An  example  of  such  an  investigation  may  be  given. 
Suppose  that  a  man  during  a  period  of  rest  excretes  daily 
10  grams,  of  nitrogen,  and  that  he  then  does  100,000  Kgms. 
of  work,  and  during  the  next  three  days  the  excretion  of 
nitrogen  is  raised  2  grams,  above  the  10  per  diem.  This 
means  that  2  x  6*25  =  12-5  grams,  of  proteid  has  been  decom- 
posed. Now  the  amount  of  energy  which  can  be  liberated 
from  1  gram,  of  proteid  has  been  found  to  be  equivalent  to 
1738  Kgms.  (kilogrammetres),  and  therefore  the  12*5  grams, 
decomposed  in  the  experiment  is  sufficient  to  yield  21,635 
Kgms.  of  energy,  about  20  per  cent,  of  the  total  energy 
expended  in  the  work.  The  rest  of  the  energy  must  be 
derived  from  the  fats  and  carbohydrates. 

5.  A  study  of  the  ordinary  diet  of  men  doing  muscular 


THE   TISSUES  73 

work  corroborates  the  conclusions  arrived  at  by  an  examina- 
tion of  the  excreta.  In  this  country  the  diet  of  a  labourer 
consists  of  something  like  the  following  proportions  of  food 
constituent  s : — 

Amount.  Yielding  Calories. 

Proteids          ...  130  533 

Fats        .         .          .          .  100  930 

Carbohydrates         .          .  500  2050 

The  energy  is  here  expressed  in  heat  units,  Calories — the 
amount  of  heat  required  to  raise  1  kilogramme  of  water 
through  1  degree  Centigrade.  Of  the  total  3513  Calories  of 
energy  daily  taken  in  the  food,  only  15  per  cent,  is  derived 
from  proteids,  the  rest  comes  from  the  carbohydrates  and  fats. 
The  same  is  found  to  be  the  case  in  the  diet  of  many  other 
animals,  such  as  the  horse. 

Thus  during  muscular  work  the  three  great  constituents 
of  the  body  and  of  the  food — proteids,  fats,  and  carbo- 
hydrates— are  broken  down  to  liberate  their  energy,  and 
apparently  the  muscle  tends  to  use  the  non-nitrogenous  fats 
and  carbohydrates  in  preference  to  the  proteids.  Only  when 
forced  to  do  so  does  it  take  a  large  proportion  of  its  energy 
from  these  substances. 

It  may  be  urged  that  in  athletic  training  proteids  must 
be  a  source  of  energy,  since  experience  has  taught  that 
they  are  of  such  value.  But  their  great  value  is  as  material 
from  which  the  energy-liberating  machine,  the  muscles,  can 
be  built  up  and  increased,  so  that  it  can  dispose  of  larger  and 
larger  quantities  of  food. 

Muscle  then  is  a  machine  which  has  the  power  of  libe- 
rating energy  from  proteids,  fats,  and  carbohydrates,  but  it 
uses  proteids  more  especially  in  construction  and  repair. 

The  muscles  liberate  energy  from  these  substances  by 
breaking  them  down  into  simpler  molecules  just  as  a  blow 
causes  the  disintegration  of  nitre-glycerine  and  liberates  its 
stored  energy.  There  is  not  such  a  direct  oxidation  as  occurs 
in  the  coals  in  the  furnace  of  an  engine,  for  if  this  were  so, 
the  consumption  of  oxygen  would  always  be  equivalent  to 
the  elimination  of  carbon  dioxide  and  the  other  products  of 
disintegration.  It  has,  however,  been  shown  that  a  frog, 


74  HUMAN  PHYSIOLOGY 

deprived  of  all  free  oxygen  by  placing  it  in  the  receiver  of 
an  air  pump,  and  then  transferred  through  mercury  to  an 
atmosphere  of  nitrogen,  still  continues  to  produce  carbon 
dioxide.  This  means  that  its  oxygen  must  be  intramolecular, 
must  be  in  the  muscle  molecule,  like  the  oxygen  of  nitro- 
glycerine. Probably  the  presence  of  this  oxygen  is  one  of 
the  causes  of  the  instability  of  the  molecule. 

The  muscle  then  takes  these  substances  into  itself — makes 
them  part  of  its  molecule — assimilates  them  before  breaking 
them  down.  It  is  not  necessary  to  suppose  that  all  the 
substances  are  equally  intimately  associated  with  the  muscle 
protoplasm.  In  all  probability  the  proteid  becomes  much 
more  truly  a  part  of  the  muscle  than  the  carbohydrates  and 
fats,  but  with  each  one  of  them  it  is  essential  that  it  should 
come  into  the  domain  of  the  muscle  and  not  simply  remain 
in  the  blood  and  lymph,  in  which  it  cannot  be  used. 

5.  DEATH  OF  MUSCLE. 

The  death  of  the  muscle  is  not  simultaneous  with  the 
death  of  the  individual.  For  some  time  after  somatic  death 
the  muscles  remain  alive  and  are  capable  of  contraction 
under  stimulation.  Gradually,  however,  their  irritability 
diminishes  and  finally  disappears.  They  are  then  dead, 
and  necrobiotic  changes  begin.  The  first  of  these — Rigor 
Mortis — is  a  disintegrate  chemical  change  whereby  carbon 
dioxide  and  sarcolactic  acid  are  set  free,  and  at  the  same 
time  the  soluble  myosinogen  changes  to  the  insoluble  myosin 
and  the  muscle  becomes  contracted,  less  extensile,  less  elastic, 
and  more  opaque.  The  contraction  is  a  feeble  one,  and  since 
it  affects  flexors  and  extensors  equally,  it  is  not  generally 
able  to  alter  the  position  of  the  limbs,  although  it  may  some- 
times do  so.  As  these  changes  occur,  heat  is  evolved  and  the 
muscles  become  warmer. 

The  time  of  onset  of  rigor  varies  with  the  condition  of  the 
muscles.  If  they  have  been  very  active  just  before  death, 
stiffening  tends  to  appear  rapidly. 

It  lasts  for  a  period  which  varies  with  the  species  of 
animal  and  with  the  condition  of  the  muscles,  and  as  it 
disappears  the  muscles  again  become  soft,  and  the  body 


THE   TISSUES  75 

becomes  limp.  In  all  probability  this  latter  change  is  due 
to  a  solution  of  the  coagulated  proteid  by  the  enzyme 
of  the  stomach — pepsin — which  seems  to  exist  in  all  the 
tissues.  This  can  act  only  in  the  presence  of  an  acid,  and 
the  appearance  of  sarcolactic  acid,  therefore,  allows  it  to  come 
into  play. 

II.  NERVE. 

It  is  through  the  nerves  that  our  surroundings  act  upon 
us,  and  through  nerves  that  our  muscles  are  made  to  respond 
appropriately  to  the  surrounding  conditions. 

1.  Structure  and  Development. 

In  unicellular  organisms  changes  in  the  surroundings 
act  directly  on  the  cell  protoplasm,  e.g.  an  amoeba,  when 
touched,  draws  itself  together.  But, 
even  in  these  simplest  organisms, 
certain  kinds  of  external  conditions  will 
produce  one  kind  of  change,  while  others 
will  produce  a  different  one,  as  has 
been  shown  in  considering  unilateral 
stimulation  (p.  16).  Among  unicellular 
organisms  somewhat  higher  than  amoeba 
— the  infusoria — animals  are  found  in 
which  the  cell  is  differentiated  into  a 
receiving  and  reacting  part.  Poterio- 
dendron,  a  little  infusorion  sitting  in 
a  cup-like  frame,  consists  of  a  long  pro-  FlG-  33.— Poteriodendron 

.,.  ,.  f  r,  to  illustrate  first  stage 

cess  or  cilmm  extending  up  ironi  the  in  the  evo]ution  of  a 
cell  body,  while  a  contractile  myoid  neuro-muscuiar  system, 
attaches  the  cell  body  to  the  floor  of 
the  cup.  When  the  cilium  is  touched  the  myoid  con- 
tracts, and  draws  the  creature  into  the  protection  of  its 
covering. 

But  in  more  highly  organised  animals,  where  the  reaction 
has  to  be  more  definitely  appropriate  to  the  surrounding 
conditions,  and  where  the  complexity  of  the  mechanism 
involved  is  greater,  there  is  a  development  by  which  special 
conditions  at  special  parts  of  the  surface  each  lead  to  a 


76  HUMAN  PHYSIOLOGY 

special  reaction.  This  is  brought  about  by  the  establishment 
of  what  may  be  compared  to  a  series  of  shunting  stations 
between  the  receptive  mechanism  on  the  surface  and  the 
reacting  mechanism,  the  muscles,  glands,  &c.  To  form  this 
a  part  of  the  original  covering  of  the  embryo  sinks  inwards  as 
a  canal  composed  of  the  surface  cells,  and  these  cells  remain 
in  functional  connection  with  the  surface  on  the  one  hand 
and  with  the  reacting  structures  on  the  other.  At  first  the 
cells  forming  this  tube  are  undifferentiated  and  alike,  but 
some  throw  out  processes  towards  the  surface  and  others 
towards  the  reacting  structures,  and  they  are  connected, 
not  by  actual  continuity,  but  by  coming  in  close  relationship 
to  one  another  in  a  series  of  branching  processes,  forming  a 
synapsis  (Fig.  34). 

Each  of  the  units  so  formed  has  been  called  a  neuron ; 
and  a  neuron  may  be  defined  as  one  of  the  cells,  with  all 


. 


Fia.  34.— To  show  a  receiving  (c)  and  a  reacting  Neuron  (a),  each  with 
dendrites  at  its  extremities,  and  their  connection  to  one  another 
through  a  Synapsis  (6).  , » 

its  processes,  which  build  up  the  narvous  $ystem.  These 
neurons  may  be  divided  into  the  deceiving  and  reacting 
series,  but  in  structure  they  are  alike.  The  shape  and 
characters  of  the  cells,  and  their  position  upon  the  processes 
of  the  neuron — the  fibre — varies  greatly,  but  they  have  all 
the  following  characters  in  common : — They  are  nucleated 
protoplasts,  the  protoplasm  of  which  shows  a  well-marked 
network,  in  the  meshes  of  which  a  material  which  stains 
deeply  with  basic  stains,  and  which  seems  to  be  used  up 
during  the  activity  of  the  neuron,  may  accumulate.  The 
granules  formed  of  this  material  are  generally  known  as 
Nissl's  granules  (Fig.  35). 

All  these  cells  give  off  at  least  one  process,  which  continues 
for  some  distance,  as  the  axon  or  axis  cylinder  of  a  nerve 
fibre.  Frequently  other  processes  are  given  off,  which  may 
either  pass  away  as  fibres,  or  may,  while  still  in  close 
proximity  to  the  cell,  form  a  branching  system  of  dendrites. 


THE  TISSUES 


77 


The   axons   end   in   much   the   same   manner,   so    that   all 
the  processes  are  essentially  the  same.     These  processes  are 


FIG.  35. — (a)  A  Nerve  Cell  with  Nissl's  granules ;  (b)  a  similar  cell 
showing  changes  on  section  of  its  axon. 

fibrillated,  and  the  fibrillse  may  be  traced  through  the  proto- 
plasm of  the  cells  (Fig.  36).     In  many  cases  the  dendrites 


FIG.  36. — A.  Nerve  Cell  highly  magnified  to  show  passage  of  processes 
through  the  protoplasm. 

show  little  buds  or  gemmules  upon  their  course,  and, 
according  to  some  observers,  it  is  through  these  that  one 
neuron  is  brought  into  definite  relationship  at  one  time  with 


78  HUMAN  PHYSIOLOGY 

one  set  of  neurons,  and  at  another  with  other  adjacent 
neurons.  There  is  also  some  evidence  that  the  dendrites 
as  a  whole  may  expand  and  contract,  and  thus  become 
connected  with  those  of  adjacent  neurons. 

Axon. — The  axis  cylinder  process,  as  it  passes  away  from 
the  cell,  becomes  a  Nerve  Fibre,  and  acquires  one  or  two 
coverings. 

1.  A  thin  transparent  membrane  is  present  in  all  peripheral 
nerves,  and  has  been  called  the  neurilemma.     Between  it  and 
the  axis  cylinder  a  series  of  nuclei  surrounded  by  a  small 
quantity   of  protoplasm,  forming  the   so-called   nerve  cor- 
puscles, is  found  at  intervals.     The  mode  of  origin  of  these 
is   unknown.     Fibres   with   only  this   sheath  have   a  grey 
colour,  and  may  be  called  non-medullated  fibres.     They  are 
abundant  in  the  visceral  nerves. 

2.  A  thick  white  sheath — the  medullary  sheath  or  white 
sheath  of  Schwann — which  gives  the  white  colour  to  most  of 


Fio.  37.— Pieces  of  two  white  Nerve  Fibres. 

the  nerves  of  the  body,  appears  somewhat  late  in  the  develop- 
ment of  many  nerve  fibres.  It  lies  between  the  primitive 
sheath  with  the  nerve  corpuscles  and  the  axis  cylinder.  It 
is  not  continuous,  but  is  interrupted  at  regular  intervals  at 
the  nodes  of  Ranvier  (Fig.  37).  It  is  composed  of  a  sponge- 
work  or  felt-work  of  a  horn-like  material — neuro-keratin — 
the  meshes  of  which  are  filled  with  a  peculiar  fatty  material, 
from  which  certain  chemical  substances  have  been  isolated, 
the  relationships  of  which  to  one  another  are  little  under- 
stood. The  most  abundant  of  these  has  been  called  pro- 
tagon.  It  yields  stearic  acid ;  hence  it  is  allied  to  the  fats, 
and  it  contains  nitrogen  and  phosphorus.  Its  constitution 
is  not  known.  Along  with  protagon,  or  as  a  result  of  its  de- 
composition, lecithin  occurs.  This  is  a  fat  in  which  one  of 
the  acid  radicles  is  replaced  by  phosphoric  acid  linked  to 
cholin — Hydroxyethyl-trirnethyl-ainmonium-hydroxide. 


THE   TISSUES  79 

r  Fatty  acid. 
Glycerol  J  Fatty  acid. 

{  Phosphoric  acid. 


Cholin. 


H    H 


HO— C— C- 


OH 

-  N 


Hydro  xyethyl. 


trimethyl  ammonium 


hydroxide. 

The  chief  interest  of  cholin  is  that  it  is  toxic,  and  some 
of  the  symptoms  occurring  in  degenerative  changes  of  the 
nervous  system  may  be  due  to  its  presence.  It  is  closely 
allied  to  inuscarin,  a  very  powerful  vegetable  poison. 

Cholesterin  may  also  be  obtained  in  considerable  amounts 
from  the  fatty  substance  of  the  white  sheath.  Like  the 
glycerine  of  ordinary  fats,  it  is  an  alcohol — C26H43,OH — and 
it  is  capable  of  linking  with  fatty  acids.  It  is  very  soluble  in 
hot  alcohol,  and  crystallises  out  on  cooling  in  characteristic 
square  plates,  with  a  notch  out  of  one  corner. 

The  nerve  fibres  run  together  in  bundles  to  constitute  the 
nerves  of  the  body,  and  each  bundle  is  surrounded  by  a  dense 
fibrous  sheath,  the  perineurium.  When  a  bundle  divides, 
each  branch  has  a  sheath  of  perineurium,  and  in  many  nerves 
this  sheath  is  continued,  as  the  sheath  of  Henle,  on  to  the 
single  fibres  which  are  ultimately  branched  off  from  the 
nerve. 

2.  Physiology. 

1.  Inter-relationship  of  Neurons. — The  neurons  form  a 
most  intricate  labyrinth  throughout  all  parts  of  the  body, 
and  more  especially  throughout  the  central  nervous  system. 

By  their  dendritic  terminations  each  is  brought  into  rela- 
tionship with  many  others,  and  hence  there  is  a  continued 
interaction  between  them,  the  activity  of  any  one  influencing 
the  activity  of  many  others.  In  this  way  the  constant 
activity  of  the  nervous  system,  which  goes  on  from  birth  to 
death,  during  consciousness  and  in  the  absence  of  conscious- 
ness, is  kept  up. 

It  is  unnecessary  and  gratuitous  to  invoke  the  conception 


8o  HUMAN  PHYSIOLOGY 

of  automatic  action  on  the  part  of  any  portion  of  the  nervous 
system.  Throughout  life  these  neurons  are  constantly  being 
acted  upon  from  without,  and  activity,  once  started  by  any 
stimulus,  must  necessarily  set  up  a  stream  of  action  which 
will  be  co-existent  with  life. 

2.  Stimulation  of  Neurons. — This  implies  that  a  neuron  is 
capable  of  stimulation,  that,  like  all  other  protoplasm,  it 
reacts  to  changes  in  external  conditions.  A  neuron  may  be 
stimulated  at  any  part,  but  it  is  usually  stimulated  from  one 
or  other  of  its  terminal  dendritic  endings,  either  by  changes  in 
the  tissues  round  about  or  by  changes  in  other  neurons.  Thus 
(Fig.  34)  a  neuron  may  be  thrown  into  action  by  changes  in 
the  tissue  at  its  extremity,  while  another  may  be  stimulated 
by  the  activity  of  the  former.  They  may  also  be  stimulated 
in  their  course,  as  is  demonstrated  by  pinching  the  ulnar 
nerve  behind  the  internal  condyle  of  the  humerus. 

Means  of  Stimulation. — Just  as  with  muscle,  so  with  nerve ; 
any  sudden  change  excites  it  to  activity — be  this  change  a 
mechanical  one,  as  in  pinching  a  nerve,  or  a  change  in  the 
temperature,  or  in  the  electric  conditions  in  its  neighbour- 
hood, or  in  the  chemical  surroundings  of  the  neuron — 
agents  which  withdraw  water,  like  glycerine,  stimulating  most 
strongly.  All  that  has  been  said  of  the  stimulation  of  muscle 
applies  to  the  stimulation  of  nerve  (see  p.  46  et  seq.). 

The  condition  of  the  neuron  modifies  the  effect  of  the 
stimulus,  and  the  condition  of  other  neurons  modifies  the 
ultimate  result  of  the  stimulus  on  the  body. 

The  excitability  of  a  neuron  is  modified  by  the  many  fac- 
tors. It  may  be  increased  by  a  slight  cooling,  but  is  de- 
creased at  lower  temperatures.  It  is  increased  by  a  warming 
up  to  a  certain  point.  Drying  at  first  increases  excitability, 
then  abolishes  it.  During  the  flow  of  an  electric  current  it 
is  increased  in  the  neighbourhood  of  the  negative  pole,  de- 
creased around  the  positive  pole,  in  the  same  way  as  in  muscle 
(see  p.  48).  It  is  influenced  by  many  chemical  substances, 
some  of  which  increase  its  excitability  in  small  doses,  and 
diminish  it  in  larger  doses ;  some  again  even  in  the  smallest 
dose  depress  its  activity,  e.g.  potassium  salts.  Continued 
activity  has  no  effect  on  the  excitability  of  nerve,  and  the 
phenomena  of  fatigue  are  not  manifested  as  in  muscle. 


THE  TISSUES  81 

3.  Manifestations  of  the  Activity  of  Neurons. — So  far  as 

is  at  present  known,  the  activity  of  neurons  is  not  accom- 
panied by  any  obvious  change,  although  it  is  possible  that 
movements  of  the  dendrites  or  of  the  gemmules  upon  them 
may  occur.  The  activity  of  neurons  is  made  evident — 

1st  By  their  action  upon  other  structures,  e.g.  muscles, 
glands,  &c.,  either  (a)  directly  or  (6)  indirectly,  through  other 
neurons. 

2nd.  By  electric  changes  in  the  neuron ;  and 

3rd.  By  changes  in  the  consciousness. 

The  activity  of  the  outgoing  neurons — neurons  conducting 
impulses  from  the  central  nervous  system  to  muscles,  glands, 
&c. — is  manifested  by  changes  in  the  muscles  or  other 
structures  to  which  they  go:  while  the  activity  of  ingoing 
neurons  is  made  evident  by  their  action  on  outgoing  neurons 
to  muscles,  &c.,  and  sometimes  by  modifications  in  the  state 
of  consciousness,  which  may  be  of  the  nature  of  a  simple 
brief  sensation,  or,  by  the  implication  of  a  number  of  other 
neurons,  may  develop  into  a  series  of  changes  accompanied 
by  sensations. 

Very  interesting  results  follow  from  this  fact  that  the 
activity  of  neurons  is  made  manifest  by  changes  in  the 
structures  to  which  they  pass.  Kennedy  has  shown  that,  if 
the  nerves  to  the  flexors  and  the  nerves  to  the  extensors 
of  a  dog's  thigh  be  cut,  and  the  central  end  of  the  first 
united  to  the  peripheral  end  of  the  second,  and  vice  versa, 
co-ordinate  movements  occur,  and  that  if  that  part  of  the 
brain  which  naturally  causes  extension  be  stimulated,  flexion 
occurs.  Langley  has  demonstrated  that  if  the  vagus  which 
conducts  downwards  to  the  abdominal  viscera  be  cut,  and 
the  cervical  sympathetic  which  conducts  up  to  the  head  be 
also  cut,  and  the  central  end  of  the  vagus  united  to  the 
peripheral  end  of  the  sympathetic,  vagus  fibres  grow  out- 
wards, and  when  the  vagus  is  stimulated,  the  results  which 
naturally  follow  stimulation  of  the  sympathetic  occur. 

4.  Conduction. — It  is  found  that  if  a  neuron  is  stimulated 
at  any  one  point,  some  time  elapses  before  the  result  of  the 
stimulation  is  made  manifest,  and  that  the  farther  the  point 
stimulated  is  from  the  structure  acted  upon,  the  longer  will 
be  this  latent  period.     This   of  course   indicates  that  the 

6 


82 


HUMAN  PHYSIOLOGY 


change,  whatever  it  is,  does  not  develop  simultaneously 
throughout  the  neuron,  but,  starting  from  one  point,  be  it 
one  end  or  the  middle,  travels  or  is  conducted  along.  The 
rate  of  conduction  may  be  determined — 

~Lst.  By  stimulating  a  nerve  going  to  a  muscle  at  two 
points  at  known  distances  from  one  another,  and  measuring 
the  difference  of  time  which  elapses  between  the  contraction 


Fio.  38. — M,  Muscle  attached  to  crank  lever  marking  on  revolving  drum.  The 
secondary  circuit  of  an  induction  coil  is  connected  with  a  commutator,  with 
the  crossed  wires  removed  so  that  the  current  may  be  sent  either  through 
the  wires  going  to  the  nerve  at  A  far  from  the  muscle,  or  at  B,  a  point  at  a 
measured  distance  nearer  the  muscle.  On  the  drum,  A  represents  the  onset 
of  contraction  on  stimulating  at  A,  and  B  the  onset  on  stimulating  at  B. 
To  secure  stimulation  in  each  case  with  the  drum  in  the  same  position,  the 
make  and  break  of  the  primary  circuit  is  caused  by  the  point  K  touching 
and  quitting  the  point  P. 


resulting  from  stimulation  at  each.  (Practical  Physiology, 
Chap.  VII.)  (Fig.  38.) 

2nd.  By  taking  advantage  of  the  fact  that  the  active 
part  of  a  neuron — like  the  active  part  of  a  muscle — is 
electro-positive  or  zincy  to  the  rest,  and  by  finding  how  long 
after  stimulation  at  one  point  this  electric  change  reaches 
another  point  at  a  measured  distance  oft'.  (Fig.  39.) 

The  rate  of  conduction  varies  considerably,  everything 
stimulating  protoplasmic  activity  accelerating,  and  every- 


THE  TISSUES  83 

thing  depressing  protoplasmic  activity  diminishing  it.  Under 
normal  conditions  in  the  fresh  nerve  of  the  frog,  the  nerve 
change  travels  about  33  metres  per  second. 

Factors  modifying  conduction. — Conduction  is  modified 
by  the  temperature.  Cooling  a  nerve  lowers  its  powers  of 
conduction,  gently  heating  it  increases  it.  Various  drugs 
which  diminish  protoplasmic  activity — e.g.  chloroform- 
diminish  conduction.  The  electric  current  acts  differently 
on  conduction  and  on  excitability.  While  a  weak  current 
has  little  or  no  effect,  a  strong  current  markedly  decreases 
conductivity  round  the  positive  pole,  and  to  a  less  extent 


FIG.  39. — N,  a  piece  of  a  Nerve  connected  by  non-polarisable  electrodes 
to  the  galvanometer,  G.  By  an  induction  coil,  it  may  be  stimu- 
lated at  A.  And  when  the  nerve  impulse  reaches  a,  a  deflection 
of  the  galvanometer  needle  takes  place. 

decreases  it  at  the  negative  pole,  so  that  the  general  effect 
of  a  strong  current  is  to  decrease  conductivity. 

From  this  influence  of  the  electric  current  upon  excita- 
bility and  conductivity  certain  differences  are  to  be  observed 
in  the  effects  of  stimulating  an  exposed  nerve  with  cur- 
rents of  various  strengths  and  directions.  These  have 
been  formulated  as  Pfltiger's  Law,  but  since  they  have 
no  bearing  upon  the  stimulation  of  nerve  in  the  living 
body  they  need  not  here  be  considered.  (Practical  Physi- 
ology, Chap.  VII.) 

By  using  the  electric  changes  as  an  index  of  nerve 
action,  it  has  been  found  that  when  a  neuron  is  stimulated 


84  HUMAN  PHYSIOLOGY 

in    the    middle,    the    change    travels    in  both  directions, 

although  its  result  is  made  manifest  by  the  action  of  the 
structure  on  which  it  normally  acts.  This  two-way  con- 
duction may  also  be  demonstrated  by  the  experiment  of 
paradoxical  contraction,  in  which  by  stimulating  the  branch 
of  the  sciatic  nerve  of  the  frog  going  to  the  muscles  of 
the  thigh,  the  nerve  fibres  to  the  gastrocnemius  lying 
alongside  of  them  are  also  stimulated,  and  cause  that 
muscle  to  contract.  (Practical  Physiology,  Chap.  VIII.) 

5.  Classification  of  Neurons. — Since  a  nerve  is  normally 
stimulated  from  one  or  other  end,  and  hence  conducts  in  one 
direction,  and  since  the  passage  of  impulses  along  it  are  made 
manifest  by  changes  in  the  structure  to  which  it  goes,  it  is 
possible  to  classify  nerve  fibres  according  to  whether  they 
conduct  to  or  from  the  central  nervous  system,  and  accord- 
ing to  the  structure  upon  which  they  act. 

To  find  out  the  direction  of  conduction  and  the  special 
mode  of  action  of  any  nerve,  two  methods  of  investigation 
are  employed : — 

1st.  The  nerve  may  be  cut,  and  the  results  of  section 
studied. 

2nd.  The  nerve  may  be  stimulated,  and  the  result  of 
stimulation  noted. 

Usually  these  methods  are  used  in  conjunction ;  first,  the 
nerve  is  cut,  and  when  the  changes  thus  produced  have  been 
noted,  the  upper  end  and  the  lower  end  of  the  cut  nerve  are 
stimulated. 

It  is,  of  course,  only  if  a  nerve  is  constantly  transmitting 
impulses  that  section  reveals  any  change.  If  the  nerve  is 
not  constantly  active,  stimulation  alone  will  teach  anything 
of  its  functions. 

Outgoing  or  Efferent  Nerves. — Section  of  certain  nerves 
produce  a  change  of  action  in  muscles,  glands,  &c.,  or,  if 
the  nerve  is  not  constantly  acting,  stimulation  of  the  peri- 
pheral end  of  the  cut  nerve  causes  some  change  in  the 
activity  of  these  structures.  Stimulation  of  the  central  end 
of  such  nerves  produces  no  effect.  These  nerves  therefore 
conduct  impulses  from  the  central  nervous  system  outward. 

Many  of  these  nerves  produce  an  increase  on  the  activity 
of  the  parts  to  which  they  go,  but  others  diminish  or  inhibit 


THE  TISSUES  85 

activity.  The  former  class  may  be  called  augmentor  nerves, 
the  latter  inhibitory  nerves. 

The  augmentor  nerves  may  further  be  divided  into  groups 
according  to  the  structures  upon  which  they  act.  Those  act- 
ing on  muscle  may  be  called  motor  nerves ;  those  acting  to 
cause  secretion  from  a  gland,  secretory  nerves ;  those  acting 
to  constrict  blood  vessels,  vaso-constrictor  nerves. 

The  inhibitory  nerves  may  be  similarly  subdivided  into 
musculo-inhibitory,  secreto-inhibitory ,  and  vaso-inhibitory 
nerves. 

Ingoing  or  Afferent  Nerves. — Section  of  another  set  of 
nerves  may  produce  loss  of  sensation  in  some  part  of  the 
body.  When  the  peripheral  end  of  the  cut  nerve  is  stimu- 
lated no  result  is  obtained.  When  the  central  end  is  stimu- 
lated, sensations  or  some  kind  of  action  results.  Such  nerves 
obviously  conduct  to  the  central  nervous  system.  Those 
which,  when  stimulated,  give  rise  to  sensations  may  be  called 
sensory;  those  which  give  rise  to  some  action  are  called 
excito-reflex,  because  the  action  which  results  is  produced  by 
what  is  called  reflex  action.  As  an  example  of  such  a  nerve 
we  may  take  the  branches  of  the  fifth  cranial  nerve  which 
pass  to  the  conjunctiva  of  the  eye.  When  the  conjunctiva  is 
touched — i.e.  when  this  nerve  is  stimulated — the  orbicularis 
palpebrarum  is  brought  into  action  through  the  seventh 
cranial  nerve,  and  the  eye  is  closed.  The  conjunctival  branch 
of  the  fifth  cranial  nerve  is  thus  an  excito-motor  nerve. 

When  the  terminations  of  the  lingual  nerve  in  the  tongue 
are  stimulated  the  result  is  a  free  flow  of  saliva,  through  the 
action  of  the  seventh  nerve  and  the  secretory  branches  of  the 
glosso-pharyngeal.  The  lingual  nerve  is  thus  excito-secretory. 
Further  stimulation  of  the  nerves  from  any  part — e.g.  by  a 
mustard  blister — causes  relaxation  of  the  vessels,  and  such 
afferent  nerves  may  be  called  excito-vaso-inhibitory. 

A  consideration  of  these  various  examples  shows  that  there 
is  no  hard  and  fast  distinction  between  sensory  and  excito- 
reflex  nerves.  Many  produce  both  sensation  and  reflex  action. 
Some  may  at  one  time  produce  sensation  alone,  at  another 
reflex  action  alone. 

Many  nerves  of  the  body  contain  both  afferent  and  efferent 
nerve  fibres,  and  are  called  mixed  nerves. 


86  HUMAN  PHYSIOLOGY 

6.  The  nature  of  the  "  impulse  "  which  passes  along  a  nerve 
is  due  to  changes  in  the  axis  cylinder,  since  this  without 
its  sheath  can  conduct  impulses.     Secondly,  it  is  dependent 
on  the  vitality  of  the  nerve.    Death  of  the  nerve  at  once  stops 
the  transmission  of  an  impulse. 

We  may  at  once  dismiss  the  idea  that  the  impulse  is  due 
to  a  flow  of  electricity.  Electricity  travels  along  a  nerve 
with  a  much  higher  velocity  than  the  nerve  impulse. 

Two  possibilities  remain.  The  impulse  may  be  of  the 
nature  of  a  molecular  vibration,  such  as  occurs  in  a  stetho- 
scope which  conducts  sound  vibration,  or  it  may  consist  of 
a  series  of  chemical  changes  such  as  cause  the  activity  of 
protoplasm  generally. 

In  considering  this  matter  it  must  be  remembered  that 
the  amount  of  energy  evolved  in  a  nerve  impulse  need  not 
be  great.  All  it  has  to  do  is  to  start  the  activity  of  the  part 
to  which  it  goes.  Hence  if  chemical  changes  are  the  basis 
of  the  impulse,  these  may  be  extremely  small  in  amount  and 
difficult  to  detect,  while  at  the  same  time  recuperation  may 
be  extremely  active. 

As  a  matter  of  fact,  the  evidence  of  chemical  changes  in 
nerve  fibres  is  entirely  wanting.  No  change  in  reaction, 
no  heat  production,  and  no  phenomena  of  fatigue  can  be 
demonstrated. 

7.  The  great  function  of  the  cell  is  to  preside  over  the 
nutrition  of  the  neuron.     If  any  part  of  the  neuron  is  cut  off 
from  its  connection  with  the  cell,  it  dies  and  degenerates. 
In  the  white  nerve  fibres  this  degeneration  begins  in  the 
white  sheath,  which  breaks  down  into  globules,  and  under- 
goes chemical  changes,  so  that  it  is  more  readily  stained  with 
osmic  acid.     This  is  taken  advantage  of  in  Marchi's  method 
for  tracing  degenerated  fibres.     The  change  extends  through- 
out the  whole  extent  of  the  nerve  at  the  same  time.   The  axis 
cylinder  next  breaks  up,  and  the  nerve  corpuscles  proliferate 
and  increase,  and  absorb  the  remains  of  the  white  sheath  and 
of  the  axis  cylinder,  so  that  after  about  twenty  days  nothing 
is  left  of  the  nerve  but  the  primitive  sheath,  filled  with  the 
nucleated  protoplasts.     According  to  some  observers,  after  a 
time  these  nucleated  structures  begin  to  throw  out  processes 
along  the  course  of  the  degenerated  fibre,  and  these  processes, 


THE  TISSUES  87 

joining  together,  regenerate  the  nerve,  so  that  frequently  when 
the  detached  end  is  united  to  the  central  part,  the  nerve  is 
very  rapidly  capable  of  performing  its  functions  again.  Other 
investigators  maintain  that  the  regeneration  is  always  due  to 
outgrowths  from  the  axis  cylinder,  either  of  the  central  end 
of  the  cut  nerve  or  from  adjacent  nerves,  and  this  is  sup- 
ported by  the  fact  that  a  cut  nerve  will  grow  down  into  any 
other  nerve  with  which  it  is  connected  (see  p.  81). 

The  cell  of  the  neuron  appears  to  have  the  power  of  accu- 
mulating a  reserve  of  material  as  Nissl's  granules,  for  it 
has  been  found  that  after  continued  action  these  granules 
diminish  in  amount.  The  nucleus,  too,  would  seem  to  have 
the  power  of  giving  off  material  for  the  nourishment  of 
the  neuron,  since  in  conditions  of  excessive  activity  it  has 
been  found  shrunken  and  distorted.  Whether  the  cells  play 
any  other  part  in  the  physiology  of  the  neuron  is  not  known. 

But  the  cell  is  also  dependent  for  its  proper  nutrition  upon 
the  condition  of  the  rest  of  the  neuron.  When  the  axon  is 
cut,  the  chromatin  of  the  cell  nucleus  decreases,  and  the 
nucleus  becomes  displaced  to  one  side,  and  ultimately  the 
whole  cell  degenerates.  This  is  sometimes  called  Nissl's 
Degeneration  (see  Fig.  35). 

The  passage  of  excitation   from  one  neuron  to  others 

occupies  a  very  appreciable  time.  This  may  be  readily 
demonstrated  in  what  is  called  reflex  action,  which  may  be 
denned  as  the  response  through  outgoing  neurons  which 
follows  the  stimulation  of  ingoing  neurons.  As  examples  of 
this  the  drawing  up  of  the  leg  when  the  sole  is  tickled,  or 
the  winking  of  the  eye  when  the  eyeball  is  touched,  may  be 
taken.  In  each  of  these  the  end  of  an  ingoing  neuron  is 
excited ;  the  change  passes  in  and  sets  up  changes  in  out- 
going neurons,  which  act  upon  muscles.  In  the  case  of  the 
eye,  about  '06  second  elapses  between  the  touching  of  the 
eye  and  the  resulting  "  wink." 

Knowing  the  rate  at  which  nerve  changes  pass  along 
nerves,  and  knowing  the  length  of  the  ingoing  and  of  the 
outgoing  neurons,  the  time  taken  in  the  passage  of  the 
change  along  these  is  readily  calculated.  In  a  reflex  wink 
it  is  about  '01  second. 


88  HUMAN  PHYSIOLOGY 

Hence  only  £th  of  the  total  "  latent  time  "  of  the  reflex 
action  is  occupied  in  the  passage  of  the  change  along  the 
neurons,  and  '05  second,  or  £th  of  the  whole  is  taken  up  in 
the  passage  of  the  change  from  one  neuron  to  another. 
Obviously  the  synapsis  between  the  dendrites  of  the  neurons 
offers  a  resistance,  and  this  resistance  varies  with  the  con- 
dition of  the  neurons  involved,  possibly  with  the  condition 
of  the  dendrites  which  form  the  synapses. 

Not  only  does  the  time  of  the  resulting  action  vary  with 
the  state  of  the  neurons,  but  the  extent  also  varies.  If  the 
toe  of  a  frog  deprived  of  its  brain  is  pinched,  it  draws  up  the 
leg;  but  if  a  dose  of  strychnine  is  first  administered,  even 
touching  the  toes  causes  a  violent  spasm  of  every  muscle  in 
the  body.  If,  on  the  other  hand,  a  dose  of  bromide  of 
potassium  has  been  administered,  or  if  ice  be  put  on  the 
back  of  the  animal,  much  more  powerful  stimulation  is 
required  to  produce  any  reaction.  The  activity  of  the 
central  synapses  may  be  increased  or  diminished  in  various 
ways,  and  hence  it  is  never  easy  to  predicate  the  ultimate 
result  of  any  stimulation  of  the  nervous  system.  But,  other 
things  being  equal,  the  strength  of  stimulus  applied  to  the 
first  neuron — that  is  the  extent  of  excitation— directly  affects 
the  extent  of  the  resulting  action.  (Practical  Physiology, 
Chap.  XII.) 


THE    NEURO-MUSCULAR    MECHANISM. 

The  study  of  the  physiology  of  muscle  and  nerve  leads  to 
the  consideration  of  how  the  neuro-muscular  mechanism 
acts,  so  that  (1st),  the  various  visceral  muscles  perform 
their  functions,  and  (2nd),  so  that  the  co-relationship  of  the 
animal  with  its  surroundings  may  be  maintained.  With  the 
second  of  these  we  shall  at  present  deal,  leaving  the  former 
for  consideration  when  studying  the  physiology  of  the  viscera. 
This  interaction  of  the  body  on  the  surroundings  may 
be  considered  first  as  regards  the  simpler  reactions — the 
so-called  reflex  actions;  and  secondly  as  regards  the  more 
complex  reactions  cominonty  classified  as  voluntary  actions. 
This  neuro-muscular  mechanism  is  controlled  by  three  chains 


THE  TISSUES 


89 


or  arcs  of  neurons,  consisting  of  ingoing  neurons  on  the  one 
side,  and  outgoing  neurons  on  the  other. 

1.  Spinal  or  Peripheral  Arc— A.  Ingoing  (Fig.  40,  A).— 
These  neurons  start  in  dendritic  expansions  at  the  peri- 
phery, and  enter  the  cord  by  the  posterior  roots  of  the  spinal 
nerves.  In  these  roots  they  are  connected  with  cells  by  lateral 
branches  (see  p.  142).  When  they  enter  the  cord  they  pass 
to  the  posterior  portion,  and  divide  into  (a)  branches  running 
for  a  short  distance  down  the  cord;  (6)  branches  run- 
ning right  up  to  the  top  of  the  spinal  cord  to  end  in 


I'iC-.  40. — To  show  the  three  Arcs  in  the  Central  Nervous  System.  A,  Peripheral 
ingoing  neuron  giving  off  collaterals  in  the  cord  and  terminating  above  in  the 
nuciM  of  posterior  columns  ;  B,  peripheral  outgoing  neurons ;  C,  ingoing 
cerebra1  neurons;  D,  outgoing  cerebral  neurons,  decussating  at//  above  the 
cord  ;  E,  r'ngoing  cerebellar  neurons  ;  F,  outgoing  cerebellar  neurons. 

synapses  round  masses  of  cells — the  nuclei  of  the  posterior 
columns. 

From  these,  collateral  branches  are  given  off  which  pass 
forward,  and,  for  the  most  part,  end  in  synapses  adjacent  to 
cells  placed  in  the  front  part  of  each  side  of  the  cord. 

B.  Outgoing  (Fig.  40,  B). — From  these  cells,  fibres  are  given 
off  which  pass  out  in  the  anterior  roots  of  the  spinal  nerves 
to  muscles,  glands,  and  other  reacting  structures.  The  action 
of  these  neurons  is  controlled  and  modified  by  the  two  other 
series  of  central  neurons. 

2.  Cerebral  Arc— A.  Ingoing  (Fig.  40,  O)— (a)  Lower 
Neurons. — These  are  (i.)  ingoing  neurons  of  the  spinal  arc 
which  lead  up  to  the  top  of  the  spinal  cord  and  end  in 
synapses  in  the  nuclei  of  the  posterior  column ;  (ii.)  inter- 
mediate neurons.  These  start  from  the  cells  in  the  nuclei  of 


90  HUMAN  PHYSIOLOGY 

the  posterior  columns,  and,  crossing  the  middle  line,  run  up 
to  the  base  of  the  great  brain,  where  they  end  in  synapses 
round  other  cells  in  the  thalamus  opticus. 

(6)  Upper  Neurons. — From  these  cells,  processes  pass  up 
to  the  surface  of  the  great  brain,  to  end  in  synapses  with  the 
cells  situated  there. 

B.  Outgoing  (D). — These  start  in  the  cells  of  the  cortex 
cerebri,  and  pass  down  to  the  upper  part  of  the  spinal  cord, 
where  most  of  them  cross  and  run  down  the  lateral  column 
of  the  spinal  cord,  giving  off  collaterals  which  end  in 
synapses  round  the  cells  in  the  anterior  horn  of  grey  matter, 
from  which  the  spinal  outgoing  neurons  pass  to  the  muscles, 
&c.  Those  which  do  not  cross  run  down  the  anterior  column 
of  the  cord  for  some  distance,  and  end  by  crossing  and 
becoming  associated  with  the  cells  in  the  anterior  horn. 

3.  Cerebellar  Arc— A.  Ingoing  (E).— Some  of  the  collaterals 
of  the  spinal  ingoing  neuron  end  in  synapses  round  a  mass 
of  nerve  cells  at  the  side  of  the  grey  matter  of  the  spinal 
cord — the  cells  of  Lockhart  Clarke.  From  these  cells,  fibres 
extend  up  at  the  side  of  the  cord  to  the  lesser  brain  or 
cerebellum,  to  form,  directly  or  indirectly,  synapses  round 
the  cells  in  this  organ. 

B.  Outgoing  (F). — From  cells  near  the  surface  of  the  cere- 
bellum and  in  the  roof  nucleus  (a)  axons  extend  to  the 
medulla  oblongata,  where  they  end  in  synapses  round  a  mass 
of  cells — the  nucleus  of  Deiters.  From  these  cells,  fibres 
extend  down  the  lateral  columns  of  the  spinal  cord,  and 
give  off  collaterals  to  the  cells  of  the  anterior  horn  of  grey 
matter.  (6)  Other  neurons  pass  to  the  cerebrum. 

The  nervous  system  may  thus  be  considered  as  built 
up  of  these  three  sets  of  arcs. 

1st.  The  Spinal  arcs,  consisting  of  the  peripheral  ingoing 
neurons  and  the  peripheral  outgoing  neurons.  These  arcs 
are  not  only  at  the  level  of  the  cord  at  which  the  ingoing 
neuron  enters,  but  at  various  levels  above  and  below  this 
point. 

2nd.  The  Cerebral  arcs,  consisting  of  the  peripheral,  the 
intermediate  and  the  upper  ingoing  neurons,  and  the  central 
outgoing  neurons,  and  the  peripheral  outgoing  neurons. 

3rd.  The  Cerebellar  arcs,  consisting  of  peripheral  ingoing 


THE  TISSUES  91 

neurons,  the  cerebellar  ingoing  neurons,  the  outgoing  cere- 
bellar  neurons,  either  direct  to  the  cord  or  through  the 
cerebrum,  and  the  peripheral  outgoing  neurons. 

A.  Simple  Reactions. 

Reflex  Action. — This  has  already  been  considered  shortly 
in  dealing  with  the  stimulation  of  outgoing  neurons  through 
their  synapses  with  ingoing  neurons.  It  has  been  shown 
that  considerable  time  is  occupied  in  the  passage  of  the 
stimulus,  and  that  the  resulting  action  depends  upon  the 
condition  of  the  neurons.  In  the  ordinary  reflex  action  the 
synapses  involved  are  situated  in  the  spinal  cord. 

Spinal  Reflex  Action. — The  simplest  manifestation  of 
reflex  action  is  to  be  seen  in  the  effect  of  pinching  the  toe 
of  a  frog  in  which  the  brain  has  been  destroyed  so  that  the 
spinal  arcs  can  act  without  interference.  If  the  pinch  is 
gentle  the  foot  is  simply  drawn  up.  If  the  pinch  is  stronger 
a  more  extensive  movement  occurs,  and  if  the  leg  is  held 
firmly  the  opposite  limb  is  drawn  up.  If  a  piece  of  paper 
dipped  in  acetic  acid  is  laid  on  the  animal's  flank,  extensive 
and  well  co-ordinated  movements  are  made  with  one  or  both 
limbs  to  remove  the  irritant.  In  all  cases  the  act  is  a  definite 
and  co-ordinated  one,  bringing  about  an  appropriate,  but 
inevitable  reaction  of  the  animal  on  its  surroundings.  This 
is  due  to  the  passage  of  an  impulse  inwards,  which  sets 
up  changes  in  the  outgoing  neurons.  With  a  very  gentle 
stimulus  the  effect  manifests  itself  at  the  same  level  of  the 
cord,  but  with  more  powerful  stimuli  other  levels  of  the  cord 
are  acted  upon  and  a  more  extensive  movement  results. 
Generally  speaking,  stimulating  one  hind  limb  causes  a 
movement  first  in  the  limb  stimulated,  then  in  the  fore  limb 
of  the  same  side,  then  in  the  opposite  fore  limb,  and  lastly  in 
the  opposite  hind  limb.  (Practical  Physiology,  Chap.  XII.) 
In  the  "  spinal  dog  " — a  dog  with  the  spinal  cord  separated 
from  the  brain  by  section  in  the  dorsal  region — Sherrington 
finds  that  different  kinds  of  stimuli  produce  different  kinds 
of  result. 

In  reflex  action  the  shortest  arc  is  usually  the  line  of  least 
resistance  along  which  the  nerve-change  travels,  and  more 


92  HUMAN  PHYSIOLOGY 

distant  arcs  are  less  readily  brought  into  play.  But  in  various 
more  complex  reflex  actions  certain  special  arcs  are  associated, 
apparently  as  the  result  of  their  having  been  educated  to 
act  together,  and  whether  this  association  is  an  inherited  one, 
or  whether  it  has  been  acquired,  there  seems  to  be  a  ten- 
dency for  nerve  action,  which  has  once  travelled  by  a  definite 
route,  to  take  the  same  route  again.  Definite  channels  of 
communication  connecting  ingoing  impulses  with  definite 
outgoing  reaction  are  thus  established. 

The  spinal  reflexes  are  modified — 1st,  By  the  condition  of 
the  neurons  (p.  81).  2nd,  By  stimuli  from  adjacent  areas. 
The  reflex  act  of  sneezing,  set  up  by  stimulation  of  the 
nasal  mucous  membrane,  may  be  checked  by  firmly  pressing 
over  the  bridge  of  the  nose.  3rd,  By  the  upper  arcs. 
Spinal  reflexes  are  increased  when  the  cord  is  separated  from 
the  brain. 

Cerebral  and  Cerebellar  Reflex  Actions. — The  upper  arcs 
may  also  act  reflexly — i.e.  inevitably.  Thus  stimulating 
certain  areas  of  skin  in  the  dog  leads  to  a  reflex  scratching 
in  which  the  body  is  bent  and  balanced,  while  the  hind 
leg  performs  complex  movements.  Here  the  balancing 
action  of  the  cerebellar  arc,  as  well  as  the  directive  action  of 
the  cerebral  arc,  are  both  involved.  But  since  the  action  is 
obviously  inevitable,  it  is  classed  as  a  reflex. 

B.  More  Complex  Reactions. 

It  has  been  seen  that  the  spinal  reflexes  are  noc  absolutely 
inevitable  since  they  are  modified  by  the  condition  of  the 
nervous  structures  involved. 

When  the  more  complex  reactions,  in  which  the  cerebral 
arc  is  involved,  are  studied,  the  resulting  action  appears 
to  be  less  inevitable,  and  to  be  influenced  by  the  sensations 
and  other  changes  in  consciousness  which  accompany  it. 
It  has,  in  fact,  been  assumed  that  the  state  of  consciousness 
is  the  determining  factor  in  the  result,  and  hence  such 
actions  have  been  called  voluntary.  But  since  in  such 
conditions  as  sleep-walking  and  hypnosis  the  most  complex 
and  selective  actions  are  performed  without  the  intervention 


THE   TISSUES  93 

of  consciousness,  it  must  be  admitted  that  this  metaphysical 
phenomenon  is  not  an  integral  part  of  the  response  of  the 
nervous  system. 

On  the  other  hand,  we  know  that  the  character  of  the 
reaction  to  any  stimulus  is  largely  dependent  upon  the  state 
of  the  nervous  centres.  Just  as  a  touch  produces  a  different 
effect  in  a  frog  poisoned  with  strychnine,  and  in  one  under 
the  influence  of  bromide  of  potassium,  so  a  sudden  noise  may 
produce  a  totally  different  reaction  upon  a  person  with  a 
fatigued  brain  or  a  brain  poisoned  by  alcohol,  and  upon  one 
with  the  brain  in  a  good  state  of  nutrition. 

Not  only  does  the  temporary  state  of  nutrition  thus 
modify  the  result  of  a  stimulus,  but  the  paths  of  action 
previously  opened  and  defined  through  the  centres  also  have 
a  marked  influence.  These  paths  may  have  been  formed  in 
past  generations  and  inherited  from  the  parents.  In  young 
fowls,  as  soon  as  they  are  hatched,  the  acts  of  walking  and 
of  pecking  are  at  once  performed,  and  in  many  families 
particular  gestures  or  expressions  follow  certain  modes  of 
stimulation  in  many  different  individuals  without  the  con- 
sciousness of  the  person  being  involved.  They  are  inherited 
cerebral  reflexes.  Paths  may  also  have  been  developed 
in  the  individual  as  the  result  of  previous  activities  of  the 
nervous  mechanism.  For,  if  a  given  action  has  once  followed 
a  given  stimulus,  it  always  tends  to  follow  it  again.  This, 
in  fact,  is  the  basis  of  all  rational  education — to  open  up 
paths  in  the  nervous  system  by  which  the  most  suitable 
response  may  be  made  to  any  given  stimulus;  and  to 
prevent  the  formation  of  paths  by  which  inappropriate 
reaction  may  be  produced. 

It  is  very  important  to  recognise  clearly  the  influence 
of  these  factors  upon  the  conduct  of  the  individual — the 
nutrition  of  the  brain  at  any  moment,  and  the  inherited  and 
acquired  tendencies  in  particular  directions — since  various 
abnormalities  in  moral  and  social  conduct  may  be  explained 
by  reference  to  them,  and  since  cases  of  so-called  insanity 
are  frequently  dependent  upon  them. 

With  the  relationship  of  consciousness  to  these  reactions 
of  the  nervous  system  we  shall  not  deal  at  present.  Con- 
sciousness is  a  purely  metaphysical  conception,  and  we  do  not 


94  HUMAN   PHYSIOLOGY 

know  its  relationship  to  cerebral  action  further  than  that  we 
have  no  evidence  that  consciousness  can  manifest  itself  apart 
from  such  cerebral  activity. 

Fatigue  of  the  Neuro-Muscular  Mechanism. 

Continued  action  of  this  mechanism  leads  to  fatigue,  and 
this  may  best  be  studied  by  means  of  some  form  of  ergograph, 
an  instrument  which  enables  the  response  of  a  muscle  to 
stimuli  to  be  recorded.  If  a  muscle  be  reflexly  stimulated 
again  and  again  it  finally  ceases  to  react,  but  if  now  the  out- 
going nerve  is  stimulated  the  muscle  contracts  at  once.  This 
shows  that  fatigue  first  manifests  itself  in  the  central  synapses. 
If  the  outgoing  nerve  be  repeatedly  stimulated,  after  a  time 
the  muscle  no  longer  responds,  but  if  the  muscle  be  directly 
stimulated  it  contracts;  but  since  the  electrical  changes 
which  accompany  conduction  in  a  nerve  still  go  on  it  is 
obvious  that  the  nerve  still  acts.  It  is  therefore  the  nerve 
ending  in  the  muscle  which  is  fatigued.  Fatigue  is  due 
to  the  accumulation  of  the  products  of  the  activity  of  muscle, 
and  it  may  be  induced  in  a  normal  dog  by  injecting  the 
blood  from  a  dog  which  has  been  fatigued. 

It  is  most  important  to  keep  clearly  in  mind  the  meaning 
of  stimulus,  reaction,  and  sensation,  (a)  Stimulus  is  the 
change  in  the  surroundings  which  produces  (6)  the  Reaction, 
the  modification  in  the  action  of  some  part  of  the  body. 
(c)  Sensation  is  the  change  in  the  consciousness  which  may 
accompany  the  application  of  a  stimulus  and  the  reaction. 


SECTION    IV 

THE  SENSES 

IN  order  that  each  particular  kind  of  change  in  our  sur- 
roundings should  produce  its  appropriate  reaction,  it  is 
essential  that  the  different  kinds  of  changes  should  act  in 
different  ways — that  the  contact  of  gross  matter,  changes  of 
temperature,  vibrations  of  the  air,  vibrations  of  the  ether, 
and  various  chemical  changes  should  each  produce  a  special 
effect. 

To  secure  this,  special  peripheral  developments  of  neurons 
have  been  evolved  which  react  more  particularly  to  each  of 
these  special  kinds  of  change,  and  with  these  peripheral 
neurons  particular  parts  of  the  brain  are  connected  and 
associated,  so  that  a  reaction  to  the  various  stimuli  may 
occur.  These  reactions  may  be  accompanied  by  changes  in 
the  consciousness — by  sensations ;  and  since  our  consciousness 
is  our  instrument  of  knowledge — our  Ego — these  sensations 
appear  to  us  the  chief  and  most  important  part  of  the  action 
of  the  mechanism.  That,  in  many  reactions,  it  is  not  an 
essential  part,  we  have  already  indicated. 

A.  COMMON  SENSIBILITY. 

Before  dealing  with  the  special  senses  the  phenomena  of 
common  sensibility  may  be  considered.  By  this  is  meant  a 
series  of  somewhat  indescribable  but  quite  definite  sensations 
by  which  our  attention  is  directed  to  the  state  of  the  body 
as  a  whole  or  of  various  parts  of  the  body.  The  ordinary 
sensations  of  thirst  and  hunger  are  examples  of  these,  sensa- 
tions which,  although  due  to  changes  in  the  mouth,  throat, 
or  stomach,  give  us  information  as  to  the  general  needs  of 
the  body.  Such  sensations  may  be  considered  as  normal 
and  physiological.  But  when  abnormal  conditions  exist  in 


95 


96  HUMAN  PHYSIOLOGY 

certain  localities  they  produce  sensations  such  as  tickling, 
tingling,  &c.,  and  generally  lead  to  an  endeavour  to  remove 
the  abnormal  stimulus. 

Those  modifications  of  consciousness  produced  by  this 
mechanism  of  common  sensibility  may  be  small  or  great 
according  to  the  strength  of  the  stimulus,  and  according  to 
the  state  of  the  central  nervous  system,  and  when  excessive 
the  sensation  produced  is  called  pain.  All  pain,  since  it 
means  a  change  in  our  consciousness,  is  metaphysical.  There 
is  not  such  a  thing  as  "  physical  pain."  The  fatigue  and 
other  sequences  to  any  kind  of  pain  are  frequently  cited  as 
proofs  of  the  influence  of  mind  on  the  body.  But  we  have 
no  right  to  assume  that  they  are  caused  by  the  pain  rather 
than  by  the  physical  disturbances  in  the  nervous  system  of 
which  the  pain  is  an  accompaniment.  It  must  be  recognised 
that  pain  is  a  purely  relative  term,  and  that  conditions  which 
in  one  individual  will  cause  pain  will  not  cause  it  in  another, 
while  stimuli  which  will  produce  what  are  called  painful 
sensations  when  the  nervous  system  is  debilitated  may  give 
rise  to  sensations  not  considered  as  painful  when  the  nervous 
system  is  normal. 

The  mechanism  of  common  sensibility  and  pain  is  not 
acted  on  by  the  same  stimuli  in  all  parts  of  the  body.  The 
mouth  and  throat  are  the  parts  to  which  the  sensations  are 
referred  in  abstinence  from  fluids,  the  stomach  in  absence  of 
food.  The  intestine  appears  to  give  rise  to  sensation  only 
when  abnormally  stimulated.  In  the  skin  the  mechanism  of 
common  sensibility  is  so  closely  associated  with  the  mechanism 
of  the  tactile  and  temperature  senses  that  it  is  difficult  to 
differentiate  them.  Abnormal  stimulation  of  the  skin  pro- 
duces painful  sensations  very  readily,  while  similar  changes 
in  other  tissues — e.g.  muscles — cause  no  modification  of 
consciousness. 

The  nerve  channels  by  which  the  changes  producing 
common  sensibility  and  pain  are  transmitted  to  the  central 
nervous  system  appear  to  be  distinct  from  those  connected 
with  the  tactile  and  other  senses,  since  common  sensibility 
may  persist  while  the  tactile  sense  is  lost. 


THE   SENSES 


97 


B.  MUSCLE  AND  JOINT  SENSE. 

Closely  allied  to  the  mechanism  of  common  sensibility  is 
the  arrangement  by  which  we  are  made  aware  of  the 
movements  and  thus  of  the  position  of  various  parts  of 
the  body.  A  double  mechanism  is  here  involved — 1st, 
A  mechanism  stimulated  by  the  contraction  of  the  muscles ; 
and  2nd,  a  mechanism  acted  on  by  movements  at  the 
joints. 

(1st)  Muscle  Spindles. — Among  the  fibres  of  the  muscles 
are  found  long  fusiform  structures  containing  modified  parts 
of  the  muscle  fibres.  Into  each  spindle  a  medullated  nerve 
passes  and  breaks  up  into  a  non-medullated  plexus  round 
the  fibres.  (Fig.  41.) 

(2nd)  Organs  of  Golgi  are  swellings 
in  the  tendons  near  the  muscle  fibres 
into  which  a  medullated  fibre  enters,  and 
losing  its  white  sheath  forms  a  plexus 
of  fibrils  with  varicosities  upon  them. 

(3rd)  Varicose  terminations  of  axons 
surrounded  by  fibrous  tissues  are  found 
in  the  synovial  membranes  and  round 
joints. 

Through  these  mechanisms  informa-  Fia  4L  —"structure 
tion  is  transmitted  to  the  central  nervous 
system  as  to  the  position  and  move- 
ments of  the  various  parts,  and  this, 
although  not  necessarily  modifying  the 
consciousness,  is  of  service  in  guiding 
the  movements.  When  the  conscious- 
ness is  affected,  valuable  information  as  to  the  conditions  of 
the  surroundings  may  be  gained.  In  estimating  the  weight 
of  bodies,  these  sensations  are  much  used.  The  body  is  taken 
in  the  hand,  and  by  determining  the  amount  of  muscular 
contraction  required  to  support  or  raise  it,  the  weight  is 
estimated.  The  shape  and  size  of  objects  are  also  determined 
by  this  sense  in  conjunction  with  the  sense  of  touch.  If 
we  touch  a  book  on  the  table  we  can  form  an  idea  of  its 
shape  and  size  by  estimating  the  distance  through  which 

'  7 


of 

muscle  spindle— only 
one  fibre  represented. 
6,  capsular  space  ;  c, 
capsule;  p,  motor  ter- 
mination ;  T.S.,  sen- 
sory termination  on  the 
spindle  fibre.  (From 
EEGAUD  and  FAVRB.  ) 


98 


HUMAN   PHYSIOLOGY 


the  hand  may  be  moved  in  different  directions.  In  the  dark 
the  distance  of  objects  is  also  judged  by  estimating  the  extent 
of  movement  of  the  hand  and  arm  necessary  to  touch  them. 
The  sensations  derived  from  the  joint  and  muscle  senses  are 
often  lost  or  impaired,  and  the  condition  of  the  mechanism 
may  be  tested  by  ascertaining  the  smallest  difference  of 
weight  which  can  be  appreciated.  With  moderate  weights  of 
about  one  pound,  a  difference  of  about  5  per  cent,  can  usually 
be  detected  in  the  normal  condition.  (Experiment.) 


C.  SPECIAL   SENSES. 

I.  Tactile  Sense. 

The  Tactile  Corpuscles,  which  consist  of  a  naked  branch- 
ing varicose  termination  of  an  axon  surrounded  by  sheaths 

of  fibrous  tissue,  are  situated  in 
the  papillae  of  the  true  skin 
(Fig.  42). 

The  study  of  the  sense  of 
touch  may  be  approached  by 
touching  the  table  and  analys- 
ing the  manner  in  which  the 
conclusion  is  arrived  at  that  a 
table  is  touched.  We  conclude 
it  is  a  table  because  the  surface 
is  hard  and  smooth.  This  judg- 
ment may  or  may  not  be  correct. 
But  even  in  saying  that  the 

Fio.    42.— Simple  form  of   sensory       body     W6     touch     IS     hard     and 
nerve  termination.     In  the  tac-  .,  i          r          • 

tile   corpuscle    the    nerve  fibre      Smooth,     W6      are     also     forming 

coils  round  the  capsule  before    judgments  from  the  sensations 
entering.    (DooiEL.)  experienced.    When  we  say  the 

surface  is  hard  we  mean  it  resists  pressure,  and  when  we 
say  it  is  smooth  we  mean  that  the  skin  of  our  finger  is 
uniformly  touched  and  not  pressed  upon  at  certain  points 
as  it  would  be  if  the  surface  were  rough.  Or  it  may  be 
that  we  draw  our  finger  over  the  table  and  feel  a  con- 
tinuous contact,  and  not  the  series  of  contact  which  we 
should  experience  were  the  surface  rough.  The  determina- 


THE   SENSES  99 

tion  of  the  resistance  to  pressure  implies  the  power  of  dis- 
tinguishing differences  of  pressure.  The  determination  of  a 
continuous  contact  instead  of  a  series  of  local  contacts  im- 
plies the  power  of  determining  or  localising  the  part  or  parts 
touched,  and  the  ability  to  distinguish  between  a  continuation 
of  contact  and  a  succession  of  contacts  implies  the  power  of 
differentiating  contacts  in  time. 

The  tactile  sense  may  thus  be  best  studied  under  three 
heads : — 

1.  The  Power  of  distinguishing  Differences  of  Pressure. 
— Variations  of  pressure  in  time  and  space  are  alone  dis- 
tinguished. We  live  under  a  pressure  of  760  mm.  of  mer- 
cury, but  this  gives  rise  to  no  sensation.  Any  sudden  in- 
crease or  diminution  of  pressure,  however,  leads  to  a  marked 
change  of  sensation,  but  a  slow  change  causes  a  lesser  modi- 
fication of  consciousness.  If  a  part  of  a  body  is  uniformly 
pressed  on,  as  when  a  finger  is  immersed  in  mercury,  the 
sensation  of  pressure  is  felt  as  a  ring  at  the  surface  of  the 
mercury,  where  the  greater  pressure  of  the  mercury  joins  the 
lesser  pressure  of  the  air.  (Experiment.) 

For  these  reasons  in  testing  the  acuteness  of  the  pressure 
sense  over  the  surface  of  the  body,  these  two  factors  must 
be  kept  in  mind.  The  rate  at  which  the  pressure  is 
varied  and  the  pressure  on  adjacent  parts  of  skin  must  be 
kept  uniform.  The  part  of  the  body  being  supported  so  that 
the  muscular  sense  cannot  come  into  play,  different  weights 
may  be  applied  to  ascertain  the  smallest  difference  of  weight 
which  can  be  distinguished.  (Experiment.) 

The  pressure  sense  varies  in  different  parts  of  the  body, 
being  most  acute  where  the  nerve  terminations  are  most 
abundant,  and  less  acute  where  they  are  fewer.  Over  the 
points  of  the  finger,  with  a  weight  of  about  1  gram.,  a  differ- 
ence of  about  10  per  cent,  can  be  distinguished,  but  over  the 
leg  the  difference  must  be  much  greater.  Everything  which 
diminishes  the  activity  of  protoplasm  diminishes  the  acute- 
ness  of  the  pressure  sense,  and  therefore  when  the  skin  is 
cold  the  sense  is  much  less  acute  than  when  it  is  warm. 

Again,  since  the  sensation  is  a  modification  of  conscious- 
ness, it  is  modified  by  the  state  of  the  central  nervous 
system.  This  is  readily  fatigued,  and  hence,  if  the  tests  are 


ioo  HUMAN  PHYSIOLOGY 

applied  for  too  long  a  period  at  one  time,  the  acuteness  of 
perception  diminishes. 

2.  The    Power    of   Localising    the    Place    of   Contact.— 
Where  the  tactile  organs  are  abundant,  the  power  of  dis- 
tinguishing accurately  the  point  touched  is  more  acute  than 
in  places  where  these  are  more  scattered.     For  this  reason,  if 
two  contacts  are  made  at  the  same  time,  these  may  be  very 
close  together  in  the  former  situation,  and  each  of  them  may 
be  localised  and  felt  as  distinct  from  the  other,  whereas  in 
the  latter  situation,  they  would  be  felt  as  a  single  contact. 
The  power  is  therefore  tested  by  determining  how  near  two 
points  of  contact  may  be  brought  to  one  another,  and  still 
cause  a  double  sensation.     This  may  be  done  by  means  of 
some  form  of  aesthesiometer — e.g.  a  pair  of  compasses ;  and,  in 
using  it,  it  is  necessary  to  observe  certain  precautions.    First, 
the  two  points  must  touch  the  skin  simultaneously.    Second, 
they  must  touch  it  lightly  and  with  equal  pressure  each  time. 
Third,  they  must  not  be  worked  steadily  from  close  together 
to  further  apart,  or  vice  versa,  but  must  be  used  now  in  the 
one  way  now  in  the  other.     (Experiment.) 

In  this  way  it  is  easy  to  demonstrate  that  over  the  tips  of 
the  fingers  where  tactile  organs  are  very  abundant  the  two 
points  of  contact  will  give  rise  to  a  double  sensation  when 
no  more  than  2  mm.  apart,  while  over  the  back  of  the 
hand  they  do  not  give  a  double  sensation  till  they  are  about 
30  mm.  apart.  Over  the  thigh  and  back  they  must  be  no 
less  than  50  to  70  mm.  apart. 

Both  the  peripheral  mechanism  and  the  central  nervous 
system  are  involved  in  this  localisation,  and  hence  the  power 
varies  with  the  condition  of  the  skin  as  regards  tempera- 
ture, &c.,  and  with  the  state  of  the  nervous  system. 

3.  The  Power  of  distinguishing  Contacts  in   Time.  —  If 
the  finger  be  brought  against  a  toothed  wheel  rotated  slowly, 
the  contacts  of  the  individual  teeth  will  be  separately  felt. 
But  if  the  wheel  is  made  to  rotate  more  and  more  rapidly, 
the  separate  sensations  are  no  longer  felt,  but  a  continuous 
sense  of  contact  is  experienced.     This  indicates  that,  if  stimuli 
follow   one  another  sufficiently  rapidly,  the  sensations  pro- 
duced are  fused.     From  this  it  is  obvious  that  the  sensation 
lasts  longer  than  the  stimulus — the  contact. 


I         I        I        I        I 

L^    lX^~b<r~P^\i\ 
L  -  M    i    \  l    i   I    i 


THE   SENSES  101 

The  duration  of  the  sensation  depends  upon  the  degree 
of  stimulation  of  the  peripheral  tactile  organs.  When 
the  stimuli  are  strong,  as  in 
the  contact  of  the  finger  with 
the  rough  edge  of  a  toothed 
wheel,  the  sensations  last  a 
considerable  time,  and  thus 
stimuli,  following  one  another 

at    about    500    times    per   Second,      FlG-  43.—  Relationship   of   Sensation 
r  r  ,  .  n  to  Stimulus,  with  weak  and  strong 

cause  a  fusion  of  sensation.     On       stimuli>    stirauli  represented  by 

the  Other  hand,  if  a  Violin  String         vertical  lines—  the  strength  being 

is  made  to  vibrate   against  the       ^^ed  by  their  height.    Sensa- 

,  ...  tions  represented  by  the  curves. 

nnger,  the  stimuli  are  weak,  and 

the  resulting  sensation  of  short  duration,  and  hence  stimuli 
may  follow  one  another  with  greater  rapidity,  say  to  1200 
per  second,  and  still  be  distinguished  as  separate  sensations. 


II.  Temperature   Sense. 

Heat,  like  light,  is  physically  a  form  of  vibration  of  the 
ether.  The  temperature  sense  depends  upon  the  fact  that 
when  heat  is  withdrawn  from  the  body  we  have  one  kind 
of  sensation  which  we  call  cold,  and  when  heat  is  added  to 
our  body  another  sensation  which  we  call  hot.  This  depends 
upon  the  temperature  of  our  body  in  relationship  to  the 
surroundings,  and  not  merely  on  the  temperature  of  sur- 
rounding bodies.  If  three  basins  of  water  are  taken,  one 
very  hot,  one  very  cold,  and  one  of  medium  temperature,  and 
if  a  hand  be  placed,  one  in  the  very  hot  and  one  in  the  very 
cold  water  for  a  short  time,  and  then  transferred  to  the  basin 
with  water  at  a  medium  temperature,  the  water  will  feel  hot 
upon  the  hand  that  has  been  in  the  cold  water,  and  cold  to 
the  hand  that  has  been  in  the  hot  water.  (Experiment.) 

The  rate  at  which  heat  is  abstracted  or  added  is  the 
governing  factor  in  causing  the  sensation ;  a  sudden  change 
of  temperature  stimulates  far  more  powerfully  than  a  slow 
change.  For  this  reason  the  thermal  conductivity  of  sub- 
stances in  contact  with  the  skin  has  an  influence  upon  the 
sensation.  If  a  piece  of  iron  and  a  piece  of  flannel  side  by 
side  be  touched,  the  first  will  feel  cold,  the  second  will  not, 


102  HUMAN  PHYSIOLOGY 

because  the  first  has  high  thermal  conductivity,  the  second 
has  not,  and  thus  the  former  abstracts  heat  more  rapidly 
than  the  latter. 

Certain  parts  of  the  skin  are  stimulated  by  the  withdrawal 
of  heat  to  give  rise  to  sensations  of  cold,  while  others  are 
stimulated  by  the  addition  of  heat.  This  may  be  demon- 
strated by  taking  the  cold  point  of  a  pencil  and  passing  it 
over  the  back  of  the  hand  when  it  will  be  felt  as  cold  only  at 
certain  points ;  such  points  have  been  called  cold  spots,  while 
similar  spots  stimulated  by  the  addition  of  heat  are  called 
hot  spots.  (Experiment.) 

The  acuteness  of  the  temperature  sense  may  be  tested  by 
finding  the  smallest  difference  of  temperature  which  can  be 
distinguished.  This  may  be  done  by  taking  two  test  tubes 
in  which  thermometers  have  been  placed,  and  filling  them 
with  water  at  slightly  different  temperatures,  and  then  apply- 
ing them  to  different  parts  of  the  skin.  If  the  temperature 
is  very  low  or  very  high,  differences  of  temperature  are  not 
readily  distinguished,  and  in  fact  painful  sensations  may 
take  the  place  of  temperature  sensations.  But  between  15° 
and  50°  C.  the  power  of  distinguishing  differences  of  tempera- 
ture is  fairly  constant,  but  it  varies  in  different  parts  of  the 
body.  Over  the  cheek  as  small  a  difference  as  '2°  C.  can  be 
appreciated,  while  on  the  back  the  difference  must  be  as  great 
as  -9°  C.  (Experiment.) 

The  temperature  sense  is  independent  of  the  tactile  sense. 
The  one  may  be  lost  and  the  other  retained.  It  is  probable 
that  the  nerve  endings  in  the  deeper  layers  of  epithelium 
are  connected  with  the  temperature  sense.  But  although 
independent,  the  tactile  and  thermal  senses  influence  one 
another.  A  cold  body  placed  on  the  skin  feels  heavier  than 
a  warm  body,  as  may  be  shown  by  placing  first  a  cold  penny 
and  then  a  warm  penny  on  the  skin  of  the  forehead. 

III.  Vision. 

While  the  addition  to  and  withdrawal  from  the  surface  of 
the  body  of  the  slower  waves  of  ether  which  are  the  basis  of 
heat  act  upon  the  special  nerve  terminations  in  the  skin  to 
give  rise  to  sensations  of  heat  and  cold,  a  certain  range  of 


THE   SENSES  103 

more  rapid  vibrations  act  specially  upon  the  nerve  endings 
in  the  eye  to  produce  molecular  changes  which  in  turn  affect 
the  centres  in  the  brain  and  give  rise  to  changes  in  con- 
sciousness which  we  call  light.  The  range  of  vibrations 
which  can  act  in  this  way  is  comparatively  limited,  the 
slowest  being  about  435  billions  per  second,  the  most  rapid 
about  764  billions.  Vibrations  more  rapid  than  this,  which 
are  capable  of  setting  up  chemical  changes,  as  in  photo- 
graphy, do  not  act  upon  the  eye. 

The  action  of  light  upon  the  protoplasm  of  lower  organ- 
isms has  been  already  considered  (p.  16),  and  it  has  been 
seen  that  it  may  be  either  general  or  unilateral,  producing 
the  phenomenon  of  positive  or  negative  phototaxis.  In  more 
complex  animals  special  sets  of  cells  are  specially  set  aside  to 
be  acted  on  by  light,  and  these  are  generally  imbedded  in 
pigmented  cells  to  prevent  the  passage  of  light  through  the 
protoplasm.  Such  an  accumulation  of  cells  constitutes  an 
eye,  and  in  the  simpler  organisms  such  an  eye  can  have  no 
further  function  than  to  enable  the  presence  or  absence  of  light 
or  various  degrees  of  illumination  to  produce  their  effects. 

But  in  the  higher  animals  these  cells  are  so  arranged  that 
certain  of  them  are  stimulated  by  light  coming  in  one  direc- 
tion, others  are  stimulated  by  light  coming  in  another,  and 
while  the  former  are  connected  with  one  set  of  synapses  in 
the  brain,  the  latter  are  connected  with  another.  Thus  light 
coming  from  one  point  will  stimulate  one  set  of  cells  which 
will  excite  one  part  of  the  brain,  and  light  from  another  will 
act  upon  other  cells  which  will  excite  another  part  of  the 
brain,  and  thus  not  merely  the  degree  of  illumination  but  the 
source  of  illumination  becomes  distinguishable. 

It  is  by  this  arrangement  that  it  becomes  possible  to  form 
ideas  of  the  shape  of  external  objects.  One  directs  the  eye 
to  the  corner  of  the  ceiling,  and  the  idea  that  it  is  a  corner 
is  due  to  the  fact  that  three  different  degrees  of  illumination 
are  appreciated,  and  that  these  can  be  localised — one  above, 
one  to  the  right,  and  one  to  the  left.  One  set  of  cells  is 
stimulated  to  one  degree,  another  set  of  cells  to  another 
degree,  and  a  third  set  of  cells  to  a  third  degree ;  and  the 
different  stimulation  of  these  different  sets  of  cells  leads  to  a 
different  excitation  of  separate  sets  of  cells  in  the  brain. 


104  HUMAN  PHYSIOLOGY 

This  is  associated  with  the  perceptions  of  the  three  parts 
differently  illuminated.  From  the  previous  training  of  the 
nervous  system  we  are  taught  to  interpret  this  as  due  to  a 
corner.  But  this  interpretation  is  simply  a  judgment  based 
upon  the  sensations,  and  it  may  or  may  not  be  right.  Thus, 
instead  of  actually  looking  at  a  corner  we  may  be  looking  at 
the  picture  of  one. 

From  the  very  first  it  must  be  remembered  that  the  modi- 
fication of  our  consciousness  which  we  call  vision  is  not 
directly  due  to  external  conditions,  but  is  due  to  changes  set 
up  in  our  eye  by  these  external  states.  We  do  not  perceive 
the  object  we  are  looking  at,  but  simply  the  changes  in  our 
brain  produced  by  changes  in  the  eye  set  up  by  rays  of  light 
coming  from  the  object. 

Usually  such  changes  are  set  up  by  a  certain  range  of 
vibrations  of  the  ether,  but  they  may  be  set  up  in  other 
ways — e.g.  by  the  mechanical  stimulation  of  a  blow  on  the 
eye  ;  but,  however  set  up,  they  give  rise  to  the  same  kind  of 
changes  in  consciousness — visual  sensations.  This  fact  has 
been  formulated  in  the  doctrine  of  specific  nerve  energy, 
that  different  varieties  of  stimuli,  applied  to  the  same  organ 
of  sense,  always  produce  the  same  kind  of  sensation.  And  the 
converse  that  the  same  stimulus  applied  to  different  organs 
of  sense  produces  a  different  kind  of  sensation  for  each  also 
holds  good. 

The  visual  mechanism  not  only  gives  the  power  of  appre- 
ciating the  degree  and  source  of  illumination,  but  also  of 
appreciating  colour.  Physically  the  different  colours  are 
simply  different  rates  of  vibration  of  the  ether,  physiologi- 
cally they  are  different  sensations  produced  by  different 
modes  of  stimulation  of  the  eye.  The  slowest  visible 
vibrations  produce  changes  accompanied  by  a  sensation 
which  we  call  red,  the  most  rapid  vibrations  produce 
different  changes  which  we  call  violet.  But,  as  will  be 
afterwards  shown,  these  sensations  may  be  produced  by 
other  modes  of  stimulating  the  eye. 

The  visual  mechanism  in  this  way  gives  a  flat  picture  of 
the  outer  world,  and  from  this  flat  picture  we  have  to  form 
judgments  of  the  size,  distance,  and  thickness  of  the  bodies 
looked  at. 


THE  SENSES  105 

The  idea  of  size  depends  upon  the  extent  of  the  eye-cells 
stimulated  by  the  light  coming  from  a  body.  If  a  large 
surface  is  acted  upon,  the  body  seems  large ;  if  a  small 
surface,  the  body  seems  small.  But  the  extent  of  eye-cells 
acted  on  depends  not  merely  upon  the  size  of  the  object, 
but  also  upon  its  distance  from  the  eye.  Hence,  our  ideas  of 
size  are  judgments  based  upon  the  size  of  the  picture  in  the 
eye,  and  the  appreciation  of  the  distance  of  the  object.  The 
distance  of  an  object,  when  over  fifty  or  sixty  metres  from  the 
eye,  and  very  probably  even  when  over  as  little  as  six  metres, 
is  judged  by  the  modifications  in  its  shading  and  colour  due 
to  the  condition  of  the  atmosphere.  A  range  of  hills  will  at 
one  time  be  judged  to  be  quite  near,  at  another  time  to 
be  distant.  Since  the  estimation  of  the  size  of  an  object 
depends  upon  the  judgment  of  its  distance,  the  estimation 
we  make  of  the  size  of  such  objects  as  a  range  of  hills  is  often 
most  erroneous.  When  objects  are  nearer  to  the  eye,  a 
special  mechanism  comes  into  play  to  enable  us  to  determine 
their  distance  (see  p.  114). 

The  idea  of  thickness  or  contour  of  an  object  is  also  largely 
a  judgment  based  upon  colour  and  shading.  When  a  cube  is 
looked  at,  we  judge  that  it  is  a  cube  because  of  the  degrees  of 
illumination  of  the  different  sides— degrees  of  illumination 
which  may  be  reproduced  in  a  flat  picture  of  such  a  cube. 
When  the  object  is  near  the  eyes,  by  using  the  two  eyes 
together  a  means  of  determining  solidity  comes  into  action 
(see  p.  124). 

When  the  manner  in  which  we  gain  knowledge  of  our 
surroundings  by  vision  is  analysed,  it  must  be  admitted  that 
the  dictum  "  seeing  is  believing"  has  at  best  an  unsubstantial 
physiological  basis,  and  that  most  of  the  points  about  any- 
thing which  we  say  we  see — e.g.  its  size,  distance,  and  contour — 
are  largely  judgments  formed  by  us  upon  a  flat  picture  pro- 
duced in  the  cells  of  the  eye,  which  flat  picture  has  in  turn 
led  to  these  changes  in  our  brain  which  are  accompanied  by 
the  changes  in  our  consciousness  upon  which  our  judgment 
has  to  act. 

Any  defect  in  the  visual  mechanism  must,  and  does,  lead 
to  defects  in  the  mental  picture  formed,  and  the  accuracy  of 


106  HUMAN  PHYSIOLOGY 

the  judgment  will  depend  upon  the  accuracy  of  the  picture, 
and  upon  the  previous  experience  and  training  of  the  nerve 
structures  involved. 

But,  further,  while  the  parts  of  the  brain  connected  with 
the  visual  sense  are  usually  stimulated  by  changes  in  the 
cells  of  the  eye,  they  may  be  directly  stimulated  ;  and  when 
this  is  the  case,  a  sensation  of  light,  apparently  in  front  of 
the  eye,  is  experienced,  because  the  centres  are  naturally 
always  stimulated  by  such  illumination.  Sensations  thus 
produced  are  called  illusions,  and  they  are  well  illustrated  by 
the  flashes  of  light  before  the  eyes  which  sometimes  precede 
tin  epileptic  attack  and  which  are  caused  by  direct  irritation 
of  the  surface  of  the  brain. 

The  study  of  vision  may  be  taken  up  in  the  following 
order : — 

1.  The  mode  of  formation  of  pictures  on  the  nerve  struc- 
tures (retina)  of  the  eye. 

(1)  One  eye  (monocular  vision). 

A.  The  method  in  which  rays  of  light  are  focussed  (diop- 
tric mechanism). 

B.  The  method  in  which  the  retina  is  stimulated. 

(2)  Two  eyes  (binocular  vision). 

2.  The  conduction  of  the  nerve  impulses  from  the  retina 
to  the  brain. 

3.  The  position  and  mode  of  action  of  the  parts  of  the 
brain  in  which  the  changes  are  set  up  which  accompany 
visual  sensations  (the  visual  centre). 


1.  The  Mode  of  Formation  of  Pictures  upon  the  Retina. 

(1)  MONOCULAR  VISION. 
A.  The  Dioptric  Mechanism. 

Anatomy. — Before  attempting  to  study  the  physiology 
of  the  eye,  the  student  must  dissect  an  ox's  or  a  pig's  eye, 
and  then  make  himself  familiar  with  the  microscopic  struc- 
ture of  the  various  parts. 


THE   SENSES 


107 


C,IM 


The  eye  may  be  described  as  a  hollow  sphere  of  fibrous 
tissue  (Fig.  44),  the  posterior  part,  the  sclerotic  (Scl.),  being 
opaque ;  the  anterior  part,  the  cornea  (Cor.),  being  transparent 
and  forming  part  of  a  sphere  of  smaller  diameter  than  the 
sclerotic.  Inside  the  sclerotic  coat  is  a  loose  fibrous  layer, 
the  choroid  (Chor.),  the  connective  tissue  cells  of  which  are 
loaded  with  melanin,  a  black  pigment.  This  is  the  vascular 
coat  of  the  eye — the  larger 
vessels  running  in  its  outer 
part,  and  the  capillaries  in  its 
inner  layer.  Anteriorly,  just 
behind  the  junction  of  the 
cornea  and  sclerotic,  it  is 
thickened  and  raised  in  a 
number  of  ridges,  the  ciliary 
processes  (Oil.  J/.),  running  from 
behind  forward  and  termi- 
nating abruptly  in  front.  In 
these  the  ciliary  muscle  is 
situated.  It  consists  of  two 

Sets     of     non-Striped     muscular     FIG.  44.  —Horizontal  section  through  the 

fibres  —  first,  radiating  fibres, 
which  take  origin  from  the 
sclerotic  just  behind  the  corneo- 
sclerotic  junction,  and  run 
backwards  and  outwards  to  be 
inserted  with  the  bases  of  the 
ciliary  processes;  second,  circu- 
lar fibres  which  run  round  the  processes  just  inside  the 
radiating  fibres.  The  choroid  is  continued  forward  in  front 
of  the  ciliary  processes  to  the  pupil  as  the  iris,  and  in  it  are 
also  two  sets  of  non-striped  muscular  fibres — first,  the  circular 
fibres,  a  well-marked  band  running  round  the  pupil,  and 
called  the  sphincter  pupilLv  (Sph.  P.)  muscle  ;  second,  a  less 
well-marked  set  of  radiating  fibres,  which  are  absent  in 
some  animals,  and  which  constitute  the  dilator  pupillte 
muscle  (D.P.). 

That  part  of  the  eye  in  front  of  the  iris  is  filled  by  a 
lymph-like  fluid,  the  aqueous  humour,  while  the  part  behind 
is  occupied  by  a  fine  jelly-like  mucoid  tissue,  the  vitreous 


Left  Eye.  Cor.  .cornea ;  Scl. ,  sclerotic ; 
0[>t.  N.,  optic  nerve ;  Chor. ,  choroid  ; 
CiL  M.,  ciliary  processes  with  ciliary 
muscle ;  D. P. .dilator  pupillse  muscle ; 
Sph.  P. ,  sphincter  pupillse  muscle ; 
L. ,  crystalline  lens;  S.L.,  hyaloid 
membrane  forming  suspensory  liga- 
ment and  capsule  of  lens.  Ret., 
retina. 


io8 


HUMAN   PHYSIOLOGY 


humour.  The  vitreous  humour  is  enclosed  in  a  delicate 
fibrous  capsule,  the  hyaloid  membrane,  and  just  behind  the 
ciliary  processes  this  membrane  becomes  tougher,  and  is 
so  firmly  adherent  to  the  processes  that  it  is  difficult  to  strip 
it  off.  It  passes  forward  from  the  processes  as  the  suspensory 
ligament  (S.L.),  and  then  splits  to  form  the  lens  capsule. 
In  this  is  held  the  crystalline  lens  (L.),  a  biconvex  lens,  with 

its  greater  curvature  on  its  pos- 
terior aspect,  and  characterised  by 
its  great  elasticity.  Normally  it 
is  kept  somewhat  pressed  out  and 
flattened  between  the  layers  ot' 
the  capsule,  but  if  the  suspensory 
ligament  is  relaxed  its  natural 
elasticity  causes  it  to  bulge  for- 
ward. This  happens  when  the 
ciliary  muscle  contracts  and  pulls 
forward  the  ciliary  processes  with 
the  hyaloid  membrane. 

Between  the  hyaloid  membrane 
and  the  choroid  is  the  retina  (Ret.). 
This  is  an  expansion  of  the  optic 
nerve,  which  enters  the  eye  at  3  to 
4  mm.  to  the  inner  side  of  the 
posterior  optic  axis  (Fig.  45).  The 
white  nerve  fibres  pass  through 
the  sclerotic,  through  the  choroid, 
and  through  the  retina,  to  form  the 
white  optic  disc,  and  then  losing 
their  white  sheath,  they  spread 
out  in  all  directions  over  the  front 
of  the  retina,  to  form  its  first  layer 

—the  layer  of  nerve  fibres  (1).  These  nerve  fibres  take 
origin  from  a  layer  of  nerve  cells  (2)  behind  them,  forming 
the  second  layer.  The  dendrites  of  these  cells  arborise  with 
the  dendrites  for  the  next  set  of  neurons  in  the  third  layer, 
the  internal  molecular  layer  (3).  The  cells  of  these  neurons 
are  placed  in  the  next  or  fourth  layer,  the  inner  nuclear 
layer  (4),  and  from  these  cells,  processes  pass  backwards  to 
form  synapses  in  the  fifth,  or  outer  molecular  layer  (5),  with 


FIG.  45. — Diagram  of  a  Sec- 
tion through  the  Retina 
stained  by  Golgi's  me- 
thod. For  description, 
see  text.  (From  VAN 
GBHUCHTEN.  ) 


THE   SENSES 


109 


the  dendrites  of  the  terminal  neurons.  These  terminal 
neurons  have  their  cells  in  the  sixth  or  outer  nuclear  layer 
(6)  of  the  retina,  and  they  pass  backwards  and  end  in  two 
special  kinds  of  terminations  in  the  seventh  layer  of  the 
retina — the  rods  and  cones  (7).  These  structures  are  com- 
posed of  two  segments — a  somewhat  barrel-shaped  basal  piece, 
and  a  transparent  terminal  part  which  in  the  rods  is  cylindri- 
cal and  in  the  cones  is  pointed.  Over  the  central  spot  of  the 
eye  there  are  no  rods,  but  the  cones  lie  side  by  side,  and  the 
other  layers  of  the  retina  are  thinned  out.  The  rods  and 
cones  are  imbedded  in  the  last  or  eighth  layer  of  the  retina 


FIG.  46. — To  show  how  parallel  rays  are  brought  to  a  focus  on  the 
retina  by  refraction  at  the  three  surfaces  (a),  anterior  sur- 
face of  the  cornea ;  (6),  anterior  surface  of  the  lens ;  and 
(c),  posterior  surface  of  the  lens. 

— the  layer  of  pigment  cells,  or  tapetum  nigrum.  The 
retina  stops  abruptly  in  front  at  the  ora  serrata,  but  the 
tapetum  nigrum,  along  with  another  layer  of  epithelial  cells 
representing  the  rest  of  the  retinal  structures,  is  continued 
forwards  over  the  ciliary  processes  and  over  the  back  of  the 
iris. 

The  blood  vessels  of  the  retina  enter  in  the  middle  of  the 
optic  nerve,  and  run  out  and  branch  in  the  anterior  layer  of 
the  retina. 

The  interior  of  the  eye  may  be  examined  by  the  Ophthal- 
moscope, which  consists  essentially  of  a  small  mirror  from 
which  light  can  be  reflected  into  the  back  of  the  eye, 


i  io  HUMAN   PHYSIOLOGY 

with  a  small  hole  in  the  centre  through  which  the  observer 
can  study  the  illuminated  part  of  the  chamber.  (Experi- 
ment.) 

Physiology. — The  eye  may  be  compared  to  a  photographic 
camera,  having  in  front  a  lens,  or  lenses,  to  focus  the  light 
upon  the  sensitive  screen  behind  (Fig.  46).  The  picture 
is  formed  on  the  screen  by  the  luminous  rays  from  each 
point  outside  being  concentrated  to  a  point  upon  the  screen. 
This  is  brought  about  by  refraction  of  light  as  it  passes 
through  the  various  media  of  the  eye — the  cornea,  aqueous, 
crystalline  lens,  and  vitreous.  The  refractive  indices  of 
these,  compared  with  air  as  unity,  may  be  expressed  as 
follows : — 

Cornea  .         .     1'33          Lens      .         .     1'4~> 
Aqueous         .     1'33          Vitreous        .     1'33 

Thus  light  passes  from  a  medium  of  one  refractive  index 
into  a  medium  of  another  refractive  index — 

1.  At  the  anterior  surface  of  the  corn* -a  : 

2.  At  the  anterior  surface  of  the  lens  ; 

3.  At  the  posterior  surface  of  the  lens ; 

and  at  these  surfaces  it  is  bent.  The  degree  of  bending 
depends  upon — 1st,  The  difference  of  refractive  index.  2nd, 
The  obliquity  with  which  the  light  hits  the  surface.  This 
will  vary  with  the  convexity  of  the  lens — being  greater  the 
greater  the  convexity. 

The  posterior  surface  of  the  lens  has  the  greatest  con- 
vexity, with  a  radius  of  6  mm.  The  anterior  surface  of 
the  cornea  has  the  next  greatest,  with  a  radius  of  8  mm. 
The  anterior  surface  of  the  lens  has  the  least,  with  a  radius 
of  10  mm.  A  ray  of  light  passing  obliquely  through  these 
media  will  be  bent  at  the  three  surfaces. 

xThese  media  in  fact  form  a  compound  lens  composed  of 
a  convexo-concave  part  in  front,  the  cornea  and  aqueous, 
and  a  biconvex  part  behind,  the  crystalline  lens.  In  the  rest- 
ing normal  eye  (the  emmetropic  eye)  the  principal  focus  is 


THE   SENSES  in 

exactly  the  distance  behind  the  lens  at  which  the  layer  of 
rods  and  cones  in  the  retina  is  situated,  and  thus  it  is  upon 
these  that  light  coming  from  luminous  points  at  a  distance 
is  focussed. 

Positive  Accommodation. — If  an  object  is  brought  nearer 
and  nearer  to  the  eye,  the  rays  of  light  entering  the  eye 
become  more  and  more '  divergent, 
and  if  the  eye  be  set  so  that  rays 
from  a  distance — i.e.  parallel  rays 
— are  focussed,  then  rays  from  a  ~^^_^^  J 

nearer  object  will  be  focussed  be-  FlG>  47._T0  show  that  rays 
hind  the  retina,  and  a  clear  image  from  distant  and  near  objects 
will  not  be  formed  (Fig.  4V).  This  ^l™  °°  the  reti™ 
means  that  near  and  far  objects 

cannot  be  distinctly  seen  at  the  same  time,  a  fact  which 
can  be  readily  demonstrated  by  Scheiners  Experiment. 
(Experiment.) 

Make  two  pin  holes  in  a  card  so  near  that  they  fall  witnin 
the  diameter  of  the  pupil.     Close  one  eye  and  hold  the  holes 
in  front  of  the  other. 
Get    some    one    to 
hold  a  needle  against 
a    white     sheet     of 
paper  at  about  three 

x     L  ..      ,,  FIG.  48. — Schemer  s  Experiment. 

yards  from  the  eye, 

and  hold  another  needle  in  the  same  line  at  about  a  foot 
from  the  eye.  When  the  far  needle  is  looked  at  the  near 
needle  becomes  double. 

It  is  found  practically  that  objects  at  a  greater  distance 
than  6  metres  may  be  considered  as  "  distant,"  and  that  they 
are  focussed  on  the  retina. 

Objects  may  be  brought  nearer  and  nearer  to  the  eye,  and 
yet  be  seen  distinctly  up  to  a  certain  point,  the  near  point 
of  vision  within  which  they  cannot  be  sharply  focussed  upon 
the  retina.  This,  however,  requires  a  change  in  the  lens 
arrangement  of  the  eye,  and  this  change,  beginning 'when 
the  object  comes  within  about  6  metres  (the  far  point  of 
vision),  becomes  greater  and  greater  till  it  can  increase  no 
further  when  the  near  point  is  reached.  The  change  is 
called  positive  accommodation,  and  it  consists  in  an  in- 


H2  HUMAN   PHYSIOLOGY 

creased  curvature  of  the  anterior  surface  of  the  lens.  This 
may  be  proved  by  examining  the  images  formed  from  the 
three  refracting  surfaces  when  it  will  be  found  that  the 
image  from  the  anterior  surface  of  the  lens  becomes  smaller 
and  brighter  (Sanson's  images).  The  examination  of  these 
images  is  facilitated  by  the  use  of  the  Phacoscope.  (Experi- 
ment.) 

Positive  accommodation  is  brought  about  by  contraction 
of  the  ciliary  muscle,  which  pulls  forward  the  ciliary  pro- 
cesses to  which  the  hyaloid  membrane  is  attached,  and  thus 

relaxes  the  suspensory 
ligament  of  the  lens  and 
the  front  of  the  lens 
capsule,  and  allows  the 
natural  elasticity  of  the 
lens  to  bulge  it  forward 
(Fig.  49). 

FIG.  49.— Mechanism  of  Positive  Accommodation.         rpu:      rV,imr«    of    nosj 
The    continuous    lines  show  the   parts   in  .   P° 

negative  accommodation,  the  dotted  lines    tlV6     accommodation     is 

the  positive  accommodation.  accompanied  by  a  con- 

traction of  the  pupil 

due  to  contraction  of  the  sphincter  pupilhe  muscle.  By  this 
means  the  more  divergent  peripheral  rays  which  would 
have  been  focussed  behind  the  central  ones  are  cut  off,  and 
spherical  aberration  is  prevented. 

The  muscles  acting  in  positive  accommodation — the  ciliary 
and  sphincter  pupilLe  (Fig.  50,  C.M.  and  S.P.) — are  supplied 
by  the  third  cranial  nerve  (///.),  while  the  dilator  pupilLr  is 
supplied  by  fibres  passing  up  the  sympathetic  of  the  neck. 
The  centre  for  the  third  nerve  is  situated  under  the  aqueduct 
of  Sylvius,  and  separate  parts  preside  over  the  ciliary  muscle 
and  the  sphincter  pupillse  (see  p.  107). 

The  sphincter  centre  is  refiexly  called  into  action,  and 
the  pupil  contracted.  1st,  When  strong  light  falls  on  the 
retina  and  stimulates  the  optic  nerve.  In  this  way  the 
retina  is  protected  against  over  stimulation.  2nd,  When 
the  image  upon  the  retina  becomes  blurred  as  the  object 
approaches  the  eye.  At  the  same  time  the  centre  for  the 
ciliary  muscle  is  also  called  into  play. 

The  centre  for  dilatation  of  the  pupil  is  situated  in  the 


THE  SENSES 


D.f- 


S.CG 


medulla  oblongata.  Like  the  centre  of  the  sphincter  it 
may  be  reflexly  excited,  stimulation  of  ingoing  nerves 
causing  a  dilatation  of  the  pupil  when  the  medulla  is 
intact  (Fig.  50). 

The  dilator  fibres  pass  down  the  lateral  columns  of  the 
spinal  cord  to  the  lower  cervical  and  upper  dorsal  region 
where  they  arborise  round  cells  in  the  anterior  horn. 
From  these,  fibres  pass  by  the  anterior  root  of  the  second 
(2  D.N.),  possibly  also  of  the  first  and  third  dorsal  nerves, 
and,  passing  up  through  the  inferior  cervical  ganglion,  run 
on  to  the  superior  ganglion,  where  they  arborise  round 
cells  which 
send  axons  to 
the  Gasserian 
ganglion  of 
the  fifth  cran- 
ial nerve  (F.), 
and  from  there 

the  fibres  pass  along  the  ophthalmic 
division  and  its  long  ciliary  branches  to 
the  dilator  fibres  (D.P.). 

The  importance   of   the  course  taken 
by   these   dilator   fibres   is   considerable, 
because  diseases  of  the   spinal   cord   in       -  ^ 
the   lower    cervical    and     upper    dorsal 
region    (the     cilio-spinal    region),    and 
tumours  in  the  upper  mediastinum,  may 
interfere  with  their  acjj^,  and  by  stimu- 
lating  cause 
pupil,  or  by 
of  the  pupi 
fibres   of    th 
animals,  it  h) 


FIG.  50.— Nerve  Supply 
of  the  Intrinsic  Muscles 

^^^^^^   ^^^  of  the  Eye  (see  text). 

e  dilaB  muscle 

fff  been  demonstrated  in  all 
^gested  that  the  nerve  may  act  by 
inhibiting  the  sphincter  pupilloe,  but  the  evidence  on  this 
point  is  not  conclusive. 

The  non-striped  muscle  of  the  iris,  like  non-striped  muscle 
elsewhere,  may  act  independently  of  nerve  fibres  as  may  be 
seen  in  the  eye  of  a  decapitated  cat.  Further,  various  drugs 
seem  to  act  directly  upon  them — e.g.,  physostigmin  causes  a 
contraction,  while  atropin  causes  a  dilatation. 

8 


ID.N. 
2D.N 


Bptation   of    the 
wejBydilata\ion 


ii4  HUMAN  PHYSIOLOGY 

The  power  of  positive  accommodation  varies  at  different 
ages,  being  greatest  in  young  children,  since  in  early  life  the 
lens  is  most  convex. 

The  distance  of  the  near  point  in  cms.  is  represented  in 
the  accompanying  figure.  The  "range  of  accommodation," 


Years.     Near  point  in  cms. 

10 

7 

20 

10 

30 

14 

40 

22 

50 

40 

60 

100 

FIG.  51. — To  show  Variations  in  the  power  of  Positive  Accommodation 
throughout  life. 

i.e.  the  difference  between  the  "near  point"  and  the  "far 
point,"  steadily  decreases  as  age  advances.  After  about 
sixty  years  of  age,  on  account  of  the  flattening  of  the  lens, 
not  even  parallel  rays  can  be  focussed  except  by  using 


FIG.  52. — To  illustrate  Presbyopia,  Myopia,  and  Hypermetropia.  A,  emmetropic 
eye;  B,  presbyopic  eye;  C,  hypermetropic  eye;  D,  myopic  eye;  N.P.o. 
the  near  point,  and  F.P.'S.,  the  far  point  of  accommodation. 

positive  accommodation.     This  is  the  fully  developed  con- 
dition of  Presbyopia — old-sightedness  (Fig.  52,  B). 

Imperfections  of  the  Dioptric  Mechanism — (1)  Myopia.— 
In  certain  individuals  the  antero-posterior  diameter  of  the  eye 


THE   SENSES 


is  too  long,  and  as  a  result  parallel  rays — rays  from  distant 
objects — are  focussed  in  front  of  the  retina,  and  it  is  only 
when  the  object  is  brought  near  to  the  eye  that  a  perfect 
image  can  be  formed.  In  such  an  eye,  no  positive  accom- 
modation is  needed  till  the  object  is  well  within  the  normal 
far  point;  and  the  near  point  is  approximated  to  the  eye. 
To  enable  distant  objects  to  be  seen  it  is  necessary  to  provide 
concave  glasses  by  which  the  parallel  rays  are  rendered 
divergent  (Fig.  52,  D). 

(2)  Hypermetropia. — The  eye  of  a  considerable  number  of 
people  is  too  short  from  before  backwards,  and  thus,  in  the 
resting  state,  parallel  rays  are  focussed  behind  the  retina, 
and  to  see  even  a  distant  object  the  individual  has  to  use 
his  positive  accommodation.     As  the  object  is  approached 
to  the  eye  it  is  focussed  with  greater  and  greater  difficulty 
and  the  near  point  is  further  off  than  in  the  emmetropic 
eye  (Fig.  52,  C). 

The  long-sighted  eye  differs  from  the  slightly  presbyopic 
in  the  fact  that  not  merely  divergent,  but  also  parallel  rays, 
are  unfocussed  in  the  resting  state. 

The  condition  is  corrected  by  using  convex  glasses  which 
render  the  rays  convergent,  and,  therefore,  capable  of  being 
focussed  upon  the  retina  of  the 
shortened  eye. 

(3)  Astigmatism  is  a  defect 
due    to    unequal    curvature   of 
the  one  or  more  of  the  refract- 
ing surfaces  in  different  planes. 
If  the  vertical  curvature  of  the 
cornea    is     greater    than    the 
horizontal,  when  a  vertical  line 
is    looked    at,   horizontal  lines 
will  not  be  sharply  focussed  at 
the    same    time.      To    correct 
this  condition,  the  lesser   cur- 
vature in  any  particular  plane 
must  be  made  as  great  as  the 
greater  curvature  in  the  other 

O 

plane,  and  this  is  done  by  placing  a  cylindrical  or  part 
cylindrical  glass  in  front  of  the  eye  so  that  its  curvature  is 


FIG.  53.— To  show  the  cause  of 
Astigmatism.  A,  a  slight  curva- 
ture of  the  cornea  in  the  vertical 
plane ;  B,  more  marked  curvature 
in  the  horizontal  plane,  leading  to 
rays  from  b — a  horizontal  line  being 
focussed  in  front  of  the  retina  when 
a — a  vertical  line — is  looked  at. 


n6  HUMAN  PHYSIOLOGY 

in  front  of  the  lesser  curvature  of  the  eye  and  thus  equalises 
it  with  the  other  curvature  (Fig.  53). 


B.  Stimulation  of  the  Retina. 

1.  Reaction  to  Varying  Illuminations. — (1)  The  Blind 
Spot. — At  the  entrance  of  the  optic  nerve  the  retina  can- 
not be  stimulated  because  there  are  no  end  organs  in  that 
situation.  The  existence  of  such  a  blind  spot  may  be 


FIG.  54.— Methods  of  demonstrating  the  Blind  Spot,     a,  by  Mariotte's 
Experiment ;  6,  by  moving  a  pencil  along  a  sheet  of  paper. 

demonstrated — 1st,  By  Mariotte's  experiment,  which  consists 
in  making  two  marks  in  a  horizontal  line  on  a  piece  of  paper, 
closing  the  left  eye,  fixing  the  right  eye  on  the  left-hand 
mark  with  the  paper  held  at  a  distance  from  the  eye,  when 
both  marks  are  visible,  then  bringing  the  paper  nearer  to  the 
eye,  when  the  right-hand  mark  will  first  disappear,  and  when 
the  paper  is  brought  still  nearer  will  reappear  (Fig.  54,  a). 
(Experiment)  2nd,  By  making  a  mark  on  a  sheet  of  paper, 
and  with  the  head  close  to  the  paper  moving  the  point  of  a 
pencil  to  the  right  for  the  right  eye,  or  to  the  left  for  the 
left  eye,  when  the  point  will  disappear  and  again  reappear 
(Fig.  54,  b).  (Experiment.) 


THE  SENSES 


117 


The  eye  is  blind  for  all  objects  in  the  shaded  region.  By 
resolving  the  various  triangles  the  distance  of  the  blind  spot 
from  the  central  spot  of  the  eye  may  be  determined  (3  to 
4  mm.),  and  the  diameter  of  the  blind  spot  (1-5  mm.)  may 
also  be  ascertained. 

The  shape  of  the  blind  spot  may  be  mapped  out  by  fixing 
the  head  close  to  the  paper,  moving  the  point  of  the  pencil 
out  till  it  disappears,  and  then  moving  it  in  different  direc- 
tions and  marking  when  it  re-appears.  It  is  never  quite 
circular,  and  often  shows  rays  extending  from  its  edge  which 
are  due  to  the  blood-vessels. 
(Experiment.) 

(2)  The  Field  of  Yision.- 
The  rest  of  the  retina  forward 
to  the  ora  serrata  is  capable 
of  stimulation,  but  when  the 
eye  is  directed  forwards  the 
extent  of  retina  stimulated 
is  influenced  by  the  eyebrow 
cutting  off  rays  from  above 
and  thus  preventing  the  lower 
part  of  the  retina  being 
stimulated  to  its  margin, 
and  by  the  nose  intercepting 
rays  from  the  nasal  side,  thus 
protecting  the  outer  part  of 
the  retina. 

The  whole  range  of  objects 
which  can  be  seen  at  one  time 

constitutes  the  field  of  vision,  and  it,  may  be  indicated  by 
the  optical  angle  subtended  by  that  range  of  objects.  As 
the  distancse  from  the  eye  increases  the  field  of  vision  expands. 
It  may  be  investigated  by  the  perimeter,  an  instrument 
which  can  readily  be  made  by  describing  the  arc  of  a  circle 
upon  a  blackboard  or  sheet  of  paper,  placing  the  eye  at 
the  centre  and  directing  it  to  a  mark  in  the  middle  of  the 
circumference,  and  then  bringing  a  piece  of  chalk  inwards 
along  the  line  until  it  is  seen  (Fig.  55).  (Experiment)  On 
bringing  an  object  from  above  it  is  not  seen  till  A  is  reached, 
while  on  bringing  it  from  below  it  is  seen  at  D.  The  angle 


FIG.  55.— The  Field  of  Vision,  and  the 
method  of  investigating  it  by  the 
Perimeter.  (7,  the  point  in  the  arc 
of  a  circle  to  which  the  eye  B  is 
directed. 


HUMAN  PHYSIOLOGY 


DBG  is  larger  than  CBA.  The  angle  ABC  is  the  measure  of 
the  vertical  field  of  vision,  and  it  will  be  observed  how  it  is 
constricted  by  the  eyebrow.  The  vertical  angle  amounts  to 
about  130° ;  60°  in  the  upper  field  and  70°  in  the  lower  field. 

The  horizontal  field  worked  out  in  the  same  way  gives 
about  150°,  of  which  no  less  than  90°  are  on  the  outer  side 
and  only  60°  on  the  inner. 

Of  all  parts  of  the  retina  the  central  spot  is  the  most 
sensitive  to  differences  of  illumination. 

(3)  The  layer  of  the  retina  capable 
of  stimulation  is  the  layer  of  rods  and 
cones.     This  is  proved  by  the  experi- 
ment of  Purkinje's  images.    It  depends 
upon  the  fact  that  if  a  ray  of  light  is 
thrown  through  the  sclerotic  coat  of 
the  eye  the  shadow  of  the  blood-vessels 
stimulates  a  subjacent  layer  (Fig.  56,  c), 
and  these  vessels  appear  as  a  series  of 
wriggling  lines  on  the  surface  looked 
at.     If  the  light  is  moved  the  lines 
seem  to  move,  and,  by  resolving  the 
triangles,  it  is  possible  to  calculate  the 

FIG.  56.-To  show  that  the  distance  behind  the  vessels  of  the  part 
stimulated,  and  this  distance  is  found 
to  correspond  to  the  thickness  of  the 
retina.  (Experiment.) 

(4)  Modes    of    Stimulation.  —  The 
rods  and  cones  are  generally  stimu- 
lated by  the  ethereal  light  vibration, 
but    they    may    be    stimulated    by 

mechanical  violence  or  by  sudden  changes  in  an  electric 
current.  But,  however  stimulated,  the  kind  of  sensation  is 
always  of  the  same  kind — a  visual  sensation.  (See  p.  104.) 

(5)  Of  the  nature  of  the  changes  in  the  retina  when 
stimulated  we  know  little.  But  we  know— 

1st.  That  under  the  influence  of  light  the  cells  of  the 
tapetum  nigrum  expand  forward  between  the  rods  and  cones. 

2nd.  That  a  purple  pigment  which  exists  in  the  outer 
segment  of  the  rods  is  bleached.  Even  although  there  is 
no  purple  in  the  cones,  which  alone  occupy  the  sensitive 


hindmost  layer  of  the  re- 
tina is  stimulated.  (Pur- 
kinje's Images.}  a,  source 
of  light ;  b,  blood  -  vessel 
of  retina;  c,  shadow  of 
vessel  on  rods  and  cones ; 
d,  image  of  shadow  men- 
tally projected  on  to  the 
wall. 


THE   SENSES 


119 


central  spot  of  the  eye,  this  change  in  colour  suggests  that  a 
chemical  decomposition  accompanies  stimulation. 
3rd.  Electrical  changes.     (See  p.  83.) 

2.  The  power  of  localising  the  source  or  direction  of 
illumination  has  now  to  be  considered.  Its  acuteness 
may  be  determined  in  the  same  way  as  in  studying  the 
sense  of  touch — by  finding  how  near  two  points  may  be 
stimulated  and  still  give  rise  to  a  double  sensation.  Over 
the  central  spot  two  points  of  illumination  may  be  as 
near  to  one  an- 
other as  about 
four  micro-milli- 
metres, and  still 
two  sensations  be 
experienced.  This 
is  determined  by 
finding  the  small- 
est optical  angle 
which  can  be  sub- 
tended  by,  say 
two  stars,  with- 
out their  images 
being  fused.  This 
angle  varies  from 
seventy-three  to 


FIG.  57. — The  Power  of  localising  the  Source  of  Illu- 
mination on  different  parts  of  the  retina.  The  two 
points,  a-b,  subtended  by  the  small  angle,  fall  close 
together  at  a-b  near  the  centre  of  the  retina,  and 
still  give  rise  to  a  double  sensation ;  but  if  two 
points,  c-d,  have  their  images  formed  on  the  peri- 
phery of  the  retina,  c-d,  these  images  must  be  far 
apart  to  cause  a  double  sensation. 

fifty    seconds    in 

different  individuals,  and  this  corresponds  to  from  5 -31  to 
3*65  micros  on  the  retina  (Fig.  57).  Over  the  central  spot 
the  centres  of  the  cones  are  about  this  distance  from  one 
another,  and  it  would  seem  that,  to  get  a  double  sensation, 
two  cones  must  be  stimulated.  On  passing  to  the  more 
peripheral  part  of  the  retina,  where  the  cones  are  more  scat- 
tered, the  power  of  localising  decreases,  and  larger  and  larger 
optical  angles  must  be  subtended  by  the  two  objects — e.g. 
the  points  of  a  pair  of  compasses — in  order  that  both  may 
be  seen.  (Experiment.) 

3.  Colour  Sensation — Physics  of  Light  Vibration. — Phy- 
sically the  various  colours  are  essentially  different  rates  of 


120 


HUMAN  PHYSIOLOGY 


vibration  of  the  ether,  and  only  a  comparatively  small  range 
of  these  vibrations  stimulate  the  retina.  The  slowest  acting 
vibrations  are  at  the  rate  of  about  435  billions  per  second, 
while  the  fastest  are  not  more  than  764  billions  —  the  relation- 
ship of  the  slowest  to  the  fastest  is  something  like  four  to 
seven.  The  apparent  colour  of  objects  is  due  to  the  fact 
that  they  absorb  certain  parts  of  the  spectrum,  and  either 

transmit  onwards  other  parts, 
or  reflect  other  parts.  The 
vast  variety  of  colours  which 
are  perceived  in  nature  is 
due  to  the  fact  that  the  pure 
spectral  colours  are  modified 
by  the  brightness  of  illumina- 
tion, and  by  admixture  with 
other  parts  of  the  spectrum. 
Thus  a  surface  which  in  bright 
sunlight  appears  of  a  brilliant 
red,  becomes  maroon,  and 
finally,  brown  and  black,  as  the 

4 

Agam>     a    PUre 

of  the  retina  (Colour  Perimeter).  re  when  diluted  with  all 
A  indicates  the  extent  of  retina  fch  spectrum—  i.e.  With  white 
stimulated  by  white  and  black  ;  .  r  . 

J5,  the  part  also  capable  of  stimula-  light  —  becomes  pink  as  it  be- 
tion  by  blue  and  yellow;  and  C,  comes  less  and  less  saturated. 

of    n>y"i°i<*y  of  Colour  Sensa- 

tion.  —  1.  The  peripheral  part 
of  the  retina  is  colour  blind  —  is  incapable  of  acting  so  as  to 
produce  colour  sensations.  This  may  be  shown  by  means 
of  the  perimeter  and  coloured  chalks.  Until  the  chalk  is 
brought  well  within  the  field  of  vision  its  colour  cannot 
be  made  out.  As  the  image  of  the  chalk  travels  in  along 
the  retina  it  is  found  that  yellow  and  blue  can  be  dis- 
tinguished before  red  and  green  —  that  is,  that  there  is  a 
zone  of  retina  which  is  blind  to  red  and  green,  but  which 
can  distinguish  blue  and  yellow.  Only  the  central  part 
of  the  retina  is  capable  of  being  stimulated  by  all  colours. 
These  zones  are  not  sharply  defined,  and  vary  in  extent 
with  the  size  and  brightness  of  the  coloured  image.  (Ex- 
periment.) (Fig.  58.) 


Fio.  58.  —  Distribution  of  Colour  Sensa-     •,•    -,  .      r    , 
tion  in  relationship  to  the  surface     hght     fades- 


THE  SENSES  121 

2.  While  the  various  sensations  which  we  call  colour  are 
generally  produced  by  vibrations  of  different  lengths  falling 
on  the  retina,   colour    sensations    are    also    produced    in 
various  other  ways. 

(a)  By  mechanical  stimulation  of  the  retina.     By  pressing 
on  the  eyeball  as  far  back  as  possible  a  yellow  ring,  or  part 
of  a  ring,  may  often  be  seen.     (Experiment.) 

(b)  Simple    alternation    of  white    and    black    upon    the 
retina   may  produce  colour   sensation,  as  when   a   disc   of 
paper  marked  with  lines  is  rotated  rapidly  before  the  eye. 
(Experiment.)     (Fig.  59.) 

3.  By  mixing  different  parts  of  the  spectrum,  some  inter- 
mediate part  or  white  may  be  produced.     This  may  be  done 
by  colouring  the  surface  of  a  top  with  different  colours,  or  by 
means  of  a  sheet  of  glass  allowing  the  image  of  one  wafer  to 
fall  on  another  of  a  different  colour.     (Experiment.) 

This  means  that  by  different  modes  of  stimulation  of  the 
retina  the  same  sensation  may  be  produced.  The  sensation 
of  orange  may  be  produced  either 
when  vibrations  at  about  580 
billions  per  second  fall  on  the 
eye,  or  when  two  sets  of  vibrations, 
one  about  640  and  one  about 
560  billions,  reach  it.  By  no  pos- 
sible physical  combination  of  the 
two  is  it  possible  to  produce  the 
intermediate  rate  of  vibration. 

The  sensation  of  colour,  there- 
fore, depends  upon  the  nature  of 

.,  ,  ..  FIG.  59.— Disc  which,   when   ro- 

the  change  set  up  in  the  retina,         tated  in  a  bright  light,  gives 
and  not  upon  the  condition  pro-         impression  of  colours. 
ducing  that  change. 

4.  After  looking  for   some   time  at   any  one   colour,  on 
removing   the   colour    another    appears    in    its   place — the 
complemental  colour.     If  the  first  colour  is — 

Red,  the  second  will  be  green  blue  ; 
Orange,         ,,  „       blue ; 

Green,          „  „       pink; 

Yellow,         „  „       indigo  blue ; 

and  vice  versa.     (Experiment.) 


122  HUMAN  PHYSIOLOGY 

Theories  of  Colour  Vision. — 1.  From  consideration  of  the 
peripheral  colour  blind  zone  of  the  retina  and  of  the  more 
limited  area  giving  sensations  only  of  blue  and  yellow  when 
stimulated,  and  of  the  most  limited  central  part  giving  also 
sensations  of  red  and  green,  it  would  seem  that  some  special 
substance  or  substances  must  exist  in  each  of  these  areas 
which  by  its  or  their  stimulation  give  rise  to  the  various 
sensations. 

2.  The  phenomenon  of  complemental  colours  suggests  the 
possibility  of  there  being  one  substance  which  when  under- 
going one  change,  say  breaking  down,  produces  blue,  and 
when  undergoing  another  change,  say  building  up,  produces 
yellow,  and  another  substance  which  when  undergoing  one 
change  produces  red,  and  another  change  produces  green; 
or  that  there  are  four  different  substances,  one  when  changed 
giving  rise  to  yellow,  another  to  blue,  another  to  red,  and 
another  to  green.  When  the  substance  giving  the  sensation 
of  yellow  is  used  up,  then  the  parts  stimulated  by  the  rest  of 
the  spectrum  would  react  to  white  light  and  give  a  com- 
plemental colour,  and  so  on  through  the  other  substances. 
If  such  a  view  be  correct,  it  becomes  almost  necessary  to 
postulate  the  existence  of  another  substance  which  when 
stimulated  gives  rise  to  sensations  which  we  call  white. 

It  has  also  been  suggested  that  the  facts  may  be  explained 
on  the  assumption  that  there  are  three  substances  in  the 
retina,  one  more  especially  stimulated  by  the  red  rays  but 
also  acted  on  by  the  others,  one  chiefly  stimulated  by  the 
green  rays,  and  one  chiefly  acted  on  by  the  blue  rays.  Such 
theories,  however,  do  not  call  for  consideration  from  the 
ordinary  student. 

Colour-blindness. — While  every  one  is  colour  blind  at  the 
periphery  of  the  retina,  a  certain  proportion  of  people — about 
5  per  cent. — are  unable  to  distinguish  reds  and  green,  even 
at  the  centre  of  the  retina.  Individuals  who  manifest  this 
condition  are  often  able  to  name  colours  fairly  accurately, 
but  when  asked  to  match  a  piece  of  red  wool  from  a  number 
of  others,  they  tend  to  put  beside  it  green  wools.  The  con- 
dition is  of  great  importance  to  engine  drivers  and  seamen. 
(Experiment.) 

Colour  blindness  for  yellow  and  blue  is  very  rare. 


THE   SENSES  123 

(2)  BINOCULAR  VISION. 

By  the  fact  that  there  are  two  eyes  instead  of  only  one, 
the  following  advantages  are  attained : — 

1.  The  Field  of  Vision  is  increased,  but  is  not  doubled. 
This  is  shown  in  Fig.  60,  where  the  two  eyes  are  directed  to 
a  spot  A,  and  where  the  field  of  vision  of  the  right  eye  is 
indicated  by  continuous  lines,  that  of  the  left  eye  by  dotted 
lines.  The  two  fields  greatly  overlap,  and  the  central  part 
is  common  to  the  two  eyes.  (Experiment.)  In  animals 


\ 


;  L.FV 


FIG.  60.— The  Field  of  Vision  in  Binocular  Vision.  The  two  eyes  are  directed  to  a 
point,  A.  The  field  of  vision  of  the  right  eye  subtends  the  angle  formed  by 
the  continuous  lines,  and  that  of  the  left  that  subtended  by  the  dotted  lines. 
The  overlap  of  the  fields  is  shown  on  the  surface  looked  at  and  in  the  figure 
below,  R.F.V.  andL.F.V. 

where    the    eyes   are   placed   laterally,   the   two   fields   are 
independent. 

2.  A  mechanism  is  afforded  for  the  determination  of  the 
distance  of  near  objects,  because  as  an  object  is  approached, 
the  two  eyes  have  to  be  turned  inwards  by  the  internal  recti 
muscles,   and   by   the  degree   of  contraction   of    these,   an 
estimation   of   the   distance   is   made.     The   importance   of 
this   may   be    demonstrated    by   fixing    a    stick   vertically, 
rapidly  walking  up  to  it  with  one  eye  shut,  and  endeavour- 
ing to  touch  it  with  the  finger.     (Experiment.) 

3.  A    means   of  determining   the   solidity   of  an   object 
is   afforded,  because  if  the   object   is   near,  a   slightly   dif- 
ferent picture  is  given  on  each  retina,  and  experience  has 


124 


HUMAN   PHYSIOLOGY 


taught   us   that   this  stereoscopic   vision   indicates   solidity 
(Fig.  61). 

Corresponding  Areas  of  the  Two  Retinae.— In  order  that 


Fio.   61.— Stereoscopic 
Vision. 


Fio.  62. — Corresponding  Areas  of  the 
two  Retinae  in  Binocular  Vision. 
The  upper  and  outer  area  of  the 
right  retina  corresponds  to  the 
upper  and  inner  area  of  the  left 
retina,  and  the  other  areas  corre- 
spond as  shown  by  the  shading. 
In  each  pair  of  areas  definite  points 
correspond  with  one  another, 


with    the    two    eyes    single 

vision  may  occur,  the   eyes 

must  be  directed  to  the  same 

place,    so    that    the    image 

of  that  place  falls  on  each 

central   spot.     If  this    does 

not  occur,  double  vision  results.     If  the  central  spot  of  one 

eye  corresponds  to  the   central  spot  of  the  other,  certain 

points  in  each  retina  will  have  corresponding  points  in  the 

other   which   will   be  stimulated  by  the  same  part   of  the 

picture  when  the  eyes  are  working  together  (Fig.  62). 

Movements  of   Eyeballs. — To 

£          £  secure  this  harmonious  action  of 

1  the   two   retinae,   it  is  necessary 

that  the  eyes  should  be  freely 
movable.  Each  eye  in  its  orbit 
is  a  ball  and  socket  joint  in 
which  the  eyeball  moves  round 
every  axis  (Fig.  63).  The  axis 
of  the  eye  (a)  is  set  obliquely  to 
the  axis  of  the  orbit  (6),  and  the 
centre  of  rotation  is  behind  the 
Fm.  63.— The  left  Eyeball  in  the  centre  of  the  ball.  The  move- 
Orbit,  with  the  Muscles  acting  ments  are  produced  by  three 

upon  it.  i* 

pairs  of  muscles. 

1.  The  internal  and  external  recti  (LR.  and  Ex.  R.). 

2.  The  superior  and  inferior  recti  acting  along  the  lines 
indicated  (S.R.). 

3.  The  superior  and  inferior  obliques  acting  in  the  line  (S.ob.). 
The  internal  rectus  rotates  the  pupil  inwards. 

external  outwards. 


S.ob. 


THE   SENSES  125 

The  superior  rectus  rotates  the  pupil  upwards  and  inwards, 
inferior  (downwards    and    in- 

\     wards. 

superior  oblique  (downwards  and  out- 

l     wards. 
„     inferior       „  „  „        upwards  and  outwards. 

In  directing  the  eyes  to  the  right,  the  external  rectus  of 
the  right  eye  acts  along  with  the  internal  rectus  of  the  left. 
In  directing  the  eyes  straight  upwards,  the  superior  rectus 
and  inferior  oblique  of  each  eye  act  together ;  and  in  looking 
downwards,  the  inferior  rectus  and  superior  oblique  come 
into  play  (Fig.  64). 

When  a  distant  object  is  looked  at,  the  axes  of  the  two 
eyes  may  be  considered  as  parallel;  but  as  an  object  is 
approached  to  the  eyes,  the  axes  converge.  It  is  not  possible 
by  voluntary  effort  to  diverge  the  optic  axis  or  to  rotate 
the  eyes  round  antero-posterior  axes. 

When     the     eyes     are  Nose 

allowed   to   sweep   over    a  In  01. 

landscape  or  any  series  of  ^ 

objects,     or     when     these 

move     rapidly     past     the      £*/?.< — [ (    ^ J— >       InR. 

eyes,  or  the  eyes  rapidly 
past  them,  as  in  travelling 
by  train,  the  axes  are  -s-di,. 

directed      in      a     Series      Of     FIG.    64.— The    Movements    of    the    Pupil 

glances  to  different  points          caused  by  the  various  Muscles  of  the 

r  '  Eye.     (Eight  Eye.) 

and  the  succession  of  pic- 
tures thus  got  gives  the  idea   of  the   continuous  series  of 
objects.     This  jerking  movement  of  the  eyes  may  be  well 
seen    in  a  passenger  looking  out  of  a  railway  carriage  in 
motion. 

A  somewhat  complex  nervous  mechanism  presides  over 
these  various  movements  of  the  eyes.  All  the  muscles  are 
supplied  by  the  third  cranial  nerve,  except  the  superior 
oblique,  which  is  supplied  by  the  fourth  nerve,  and  the 
external  rectus,  which  is  supplied  by  the  sixth  nerve  (Fig. 
65  ;  see  also  Fig.  85,  p.  158). 

The  centres  for  the  third  and  fourth  nerves  are  situated 
in  the  floor  of  the  aqueduct  of  Sylvius  under  the  corpora 


126 


HUMAN   PHYSIOLOGY 


quadrigemina,  while  the  centre  for  the  sixth  is  in  the  pons 
Varolii  (Fig.  86,  p.  159,  and  Fig.  85,  p.  158).  The  various 
centres  are  joined  by  bands  of  nerve  fibres  which  pass 
between  the  sixth  and  fourth  and  third  centres,  and  in  part 
at  least  cross  the  middle  line. 

A  combined  mechanism,  each 
part  of  which  acts  harmoni- 
ously with  the  other  parts, 
thus  presides  over  the  ocular 
movements,  and  this  mechanism 
is  controlled  by  impulses  con- 
stantly received  from  the  two 
retinae,  from  the  ear  and  from 
the  brain. 

2.  Connections  of  the  Eyes 
with  the  Central  Nervous 
System. 

From  each  eye  the  optic  nerve 
extends  backwards  and  inwards 
to  join  the  other  optic  nerve 
at  the  chiasma.  From  the 
chiasma  the  two  optic  tracts 
pass  upwards  round  the  crura 
cerebri  to  end  in  two  divisions — 
1.  A  posterior  division  passing 

internal  rectus  of  the  opposite  to  the  anterior  corpora  quadri- 
side  through  the  nucleus  of  the  gemma  on  the  same  side  (Fig.  66 
*"'*-"  A.C.Q.). 

2.  An  anterior  running  to  the  geniculate  body  on  the 
posterior  aspect  of  the  thalamus  opticus  (Fig.  66,  Op.  Th.). 

A  partial  crossing  of  the  fibres  takes  place  in  the  chiasma 
— fibres  from  the  middle  and  internal  part  of  the  retina 
decussating,  those  from  the  outer  part  remaining  on  the  same 
side.  For  this  refs%n,  when  the  right  optic  tract  is  cut,  it 
leads  to  partial  blindness  of  both  retinaa — on  the  outer  part  of 
the  right  eye  and  on  the  inner  and  middle  part  of  the  left 
eye.  Thus  objects  on  the  left  side  of  the  field  of  vision  are 
not  seen. 

The  fibres  of  the  posterior  termination  of  the  optic  tract 


FIG.  65. — The  Nervous  Mechanism 
presiding  over  the  combined 
movements  of  the  two  Eyes. 
IR. ,  Internal  rectus ;  ER. ,  ex- 
ternal rectus;  CO.,  convergent 
centre  acting  on  the  internal 
recti  through  the  nuclei  of  the 
third  nerve;  S.O.,  superior  olive 
(centre  for  lateral  divergence) 
acting  on  the  external  rectus 
of  the  same  side  through  the 
nucleus  of  the  sixth,  and  on  the 


THE   SENSES 


127 


end  in  synapses  with  neurons  in  the  corpora  quadrigemina, 
and  the  fibres  of  these  neurons  pass  downwards  and  control 
the  oculo-inotor  mechanism  already  described  (Fig.  65, 
p.  126). 

The  fibres  of  the  anterior  division  make   synapses  with 
other  neurons  in   the  posterior  part  of  the  thalamus,  and 


Occ.  L. 


FlG.  66.— The  Connections  of  the  Retinae  with  the  Central  Nervous  System. 
R,  retinae;  Ch.,  chiasma  leading  to  optic  tract;  Op.  Th.,  optic  thalamus ; 
A.C.Q.,  anterior  corpora  quadrigemina;  Oc.  M.,  oculo-motor  mechanism 
(Fig.  65) ;  Occ.  L.,  occipital  lobe  of  the  cerebrum ;  Teg.)  tegmentum. 

these  neurons  send  their  fibres  backwards  to  the  occipital 
lobe  of  the  brain  where  they  connect  with  the  cortical 
neurons  (Fig.  66,  Occ.)  (see  p.  181).  ^ 

When  the  right  occipital  lobe  or  the  strand  of  fibres  leading 
to  it  is  destroyed,  blindness  on  the  outer  part  of  the  right 
retina  and  on  the  inner  and  middle  part  of  the  left  retina 
results — the  individual  is  blind  for  all  objects  in  the  left  half 
of  the  field  of  vision. 


128  HUMAN   PHYSIOLOGY 

3.  The  Visual  Centre. 

A  response  to  stimulation  on  the  part  of  the  neurons  in  the 
occipital  lobe  of  the  brain  (p.  181)  is  the  physical  basis  of  our 
visual  sensations,  and  hence  this  part  of  the  brain  is  called 
the  visual  centre.  Usually  the  visual  centre  is  stimulated 
by  changes  in  the  chain  of  neurons  passing  from  the  retina 
and  set  in  action  by  retinal  changes ;  but  direct  stimulation 
of  the  occipital  lobe  may  induce  visual  sensations,  as  is  some- 
times seen  in  the  early  part  of  an  epileptic  fit. 

The  strength  of  the  sensation  depends  upon  the  strength 
of  the  stimulus,  and  the  smallest  difference  of  sensation 
which  can  be  appreciated  is  a  constant  factor  of  the  degree 
of  stimulation.  Thus,  to  produce  a  change  in  visual  sen- 
sation, the  strength  of  the  stimulus  must  vary  by  about 
Tc75-th  of  the  stimulus. 

The  sensation  lasts  longer  tlian  the  stimulus,  and  thus, 
if  a  series  of  stimuli  follow  one  another  at  sufficiently  rapid 
intervals,  a  fusion  of  sensation  is  produced.  If  a  wheel 
rotating  slowly  is  looked  at,  the  individual  spokes  are  seen, 
but  when  it  is  going  more  rapidly,  the  appearance  of  a  con- 
tinuous surface  is  presented.  If  the  light  is  dim,  this  fusion 
takes  place  more  readily  than  when  the  light  is  bright.  From 
this  it  is  concluded  that  a  strong  stimulus  causes  a  more 
sudden  and  acute  sensation  than  a  weak  one,  and,  therefore, 
the  individual  sensations  are  distinguished. 

The  visual  centre  of  each  side  must  be  regarded  as  a  chart 
of  the  opposite  field  of  vision,  each  part  corresponding  to 
a  particular  part  of  the  field.  The  two  centres  acting 
together  give  the  whole  field  of  vision.  Since  the  blind 
spot  is  not  represented  in  the  centre,  it  is  not  perceived 
in  the  field  of  vision.  The  centre  is  said  to  rectify  the 
inverted  image  formed  on  the  retina,  but  this  simply  means 
that  as  a  result  of  experience,  we  have  learned  that  changes 
in,  say  the  lower  part  of  the  retinae  and  in  the  corresponding 
parts  of  the  visual  centres,  are  produced  by  light  from  above 
the  head. 

Since  the  retinal  changes  differ  simply  according  to  the 
degree  of  illumination  and  the  rate  of  the  ethereal  waves, 
and  since  the  part  of  the  retina  acted  on  is  determined  by 


THE   SENSES 


129 


the  direction  of  the  rays,  we  can  be  conscious  only  of 
different  changes  in  different  parts  of  the  visual  centre  depen- 
dent on  the  changes  set  up  in  the  retinae.  We  have  there- 
fore only  the  means  of  getting  a  flat  picture  of  what  we  look 
at,  but  no  special  arrangement  for  having  different  sensations 
according  to  the  distance  of  an  object  or  according  to  whether 
it  is  flat  or  in  relief.  Thus  the  means  of  determining  the 
form  and  size  of  objects  by  the  retinas  and  visual  centres  is 
very  limited. 

To  gain  knowledge  of  the  size,  distance,  or  contour  of  an 
object  we  have  to  combine  certain  other  sensations,  or  certain 
past  experiences  with  the  visual  sensation,  and  to  form  a 
judgment  or  concept  of  what  we  are  looking  at. 

It  is  not  then  wonderful  that  erroneous  judgments  are 
frequently  made.  The  possibility  of  such  may  be  indicated 
by  one  or  two  examples  of  what  are  called  modified  per- 
ceptions. 

(1)  If  an  imaginary  line  be  divided  into  two  equal  parts, 
and  if  a   series  of  dots   are   put 

along  the  one  half,  that  part  will    

appear  longer  than  the  other. 

(2)  If  a  black  wafer  on  a  white  ground,  and  a  white  wafer 
on  a  black  ground  be  looked  at,  the  latter  will  appear  larger 
than  the  former. 

(3)  If  a  square  be  ruled  with  parallel  diagonal  lines,  and 
if  short  vertical  and  horizontal  lines  be  placed  alternately 
upon  them,  they  will  no  longer  appear 

parallel. 

IY.  Hearing. 

While  through  the  sense  of  touch 
we  are  made  aware  of  differences  of 
pressure,  through  the  sense  of  hearing 
certain  vibratory  changes  of  pressure 
specially  affect  the  consciousness.  Even 
simple  organisms,  devoid  of  any  special 
organ  of  hearing,  may  be  affected  by  vibratory  changes,  and 
in  fish  it  is  difficult  to  be  certain  how  far  such  vibrations 
produce  their  effect  through  the  ear  or  through  the  body 
generally;  but  in  higher  vertebrates  it  is  chiefly  through 

9 


FIG.  67.— To  illustrate 
Optical  Illusions. 


130  HUMAN  PHYSIOLOGY 

the  ears  that  they  act.  In  these  there  is  a  special  arrange- 
ment by  which  the  vibrations  of  the  air  are  converted  into 
vibrations  of  a  fluid  in  a  sac  situated  in  the  side  of  the 
head  into  which  the  free  ends  of  neurons  project. 

In  mammals  the  organ  of  hearing  consists  of  an  external, 
a  middle,  and  an  internal  ear. 

A.  External  Ear. — The  structure  of  this  presents  no  point 
of  special  physiological  interest.     In  lower  animals  the  pinna 


LxM 


Fio.  68. — Diagram  of  the  Ear.  ExM.,  external  meatus ;  Ty. ,  tympanic  mem- 
brane; m. ,  malleus;  t.,  incus;  «.,  stapes;  /.o.,  fenestra  ovalis;  f.r.,  fenestni 
rotunda;  EnT.,  Eustachian  tube;  t>.,  vestibule;  «.c.,  semicircular  canal; 
Coch.,  cochlea. 

is  under  the  control  of  muscles,  and  is  of  use  in  determining 
the  direction  from  which  sound  comes. 

B.  Middle  Ear. — The  object  of  the  middle  ear  is  to  over- 
come the  mechanical  difficulty  of  changing  vibrations  of  air 
into  vibrations  of  a  fluid.  It  consists  of  a  chamber,  the 
tympanic  cavity,  placed  outside  of  the  petrous  part  of  the 
temporal  bone  (Fig.  68).  Its  outer  wall  is  formed  by  a 
membrane,  the  membrana  tympani  (Ty.),  which  is  attached 
to  a  ring  of  bone.  Its  inner  wall  presents  two  openings  into 
the  internal  ear — the  fenestra  ovalis  (/.o.),  an  oval  opening, 
situated  anteriorly  and  above,  and  the  fenestra  rotunda  (/.?*.), 
a  round  opening  placed  below  and  behind.  Throughout  life 
these  are  closed,  the  former  by  the  foot  of  the  stapes,  which 


THE    SENSES  131 

is  attached  to  the  margin  of  the  hole  by  a  membrane,  the 
latter  by  a  membrane.  The  posterior  wall  shows  openings 
into  the  mastoid  cells  and  presents  a  small  bony  projection 
which  transmits  the  stapedius  muscle.  The  anterior  wall 
has  above  a  bony  canal  carrying  the  tensor  tympani  muscle, 
and  below  this  the  canal  of  the  Eustachian  tube  which  com- 
municates with  the  posterior  nares  (Fig.  68,  EnT.). 

In  the  tympanic  cavity  are  three  ossicles — the  malleus  (m.), 
incus  (i.),  and  stapes  (s.),  forming  a  chain  between  the  mem- 
brana  tympani  and  the  fenestra  ovalis.  The  handle  of  the 
malleus  is  attached  to  the  membrana  tympani,  and  each  time 
a  wave  of  condensation  hits  the  membrane,  it  drives  in  the 
handle  of  the  malleus.  This,  by  a  small  process,  pushes 
inwards  the  long  process  of  the  incus  which  thrusts  the  stapes 
into  the  fenestra  ovalis,  and  thus  increases  the  pressure  in  the 
enclosed  fluid  of  the  internal  ear.  The  fenestra  rotunda  with 
its  membrane  acts  as  a  safety  valve.  The  bones  rotate 
round  an  antero-posterior  axis  passing  through  the  heads  of 
the  malleus  and  incus.  They  thus  form  a  lever  with  the 
arm  to  which  the  power  is  applied — the  handle  of  the  malleus 
— longer  than  the  other  arm.  The  advantage  of  this  is  that, 
while  the  range  of  movement  of  the  stapes  in  the  fenestra 
ovalis  is  reduced,  its  force  is  proportionately  increased. 

The  range  of  movement  is  still  further  controlled  by  the 
stapedius  muscle  which  twists  the  stapes  in  the  fenestra. 
This  muscle  seems  to  act  when  loud  sounds  fall  on  the  ear, 
and  when  its  nerve  supply,  derived  from  the  facial  nerve,  is 
paralysed,  such  sounds  are  heard  with  painful  intensity. 

If  the  membrana  tympani  is  violently  forced  outwards  by 
closing  the  nose  and  mouth  and  forcing  air  up  the  Eustachian 
tube,  the  incus  and  stapes  do  not  accompany  the  malleus  and 
membrane,  since  the  malleo-incal  articulation  becomes  un- 
locked. 

The  membrana  tympani  is  so  loosely  slung  that  it  has  no 
proper  note  of  its  own,  and  responds  to  a  very  large  range 
of  vibrations.  By  the  attachment  to  it  of  the  handle  of  the 
malleus  it  is  well  damped,  and  stops  vibrating  as  soon  as 
waves  of  condensation  and  rarefaction  have  ceased  to  fall 
upon  it.  The  tensor  tympani  muscle,  supplied  by  the 
fifth  cranial  nerve,  has  some  action  in  favouring  the  vibration 


132  HUMAN   PHYSIOLOGY 

of  the  membrane,  and  its  paralysis  diminishes  the  acuteness 
of  hearing. 

The  Eustachian  tube  has  a  double  function.  It  allows  the 
escape  of  mucus  from  the  middle  ear,  and  it  allows  the  en- 
trance of  air,  so  that  the  pressure  is  kept  equal  on  both  sides 
of  the  membrana  tympani.  Its  lower  part  is  generally  closed, 
but  opens  in  the  act  of  swallowing.  It  is  surrounded  by  an 
arch  of  cartilage  to  one  side  of  which  fibres  of  the  tensor 
palati  are  attached,  so  that  when  this  muscle  acts  in  swallow- 
ing, the  arch  of  cartilage  is  drawn  down  and  flattened,  and 
the  tube  opened  up  (Fig.  69). 

When  the  Eustachian  tube  gets  occluded,  as  a  result  of 
catarrh  of  the  pharynx,  the  oxygen  in  the  middle  ear  is  ab- 
sorbed by  the  tissues,  and  the  pressure  falls.  As  a  result,  the 
membrane  is  driven  inwards  by  the  atmospheric  pressure,  and 
does  not  readily  vibrate,  and  hearing  is  impaired. 

C.  Internal  Ear. — The  internal  ear  is  a  somewhat  complex 
cavity  in  the  petrous  part  of  the  temporal  bone,  the  osseous 
labyrinth.  It  consists  of  a  central  space, 
the  vestibule  ( V),  into  which  the  fenestra 
ovalis  opens.  From  the  anterior  part  of 
this,  a  canal  makes  two  and  a  half  turns 
round  a  central  pillar,  and  then,  turning 
sharply  on  itself,  makes  the  same  number 
of  turns  down  again,  and  ends  at  the 
fenestra  rotunda.  This  is  the  osseous 

FIG.     69.-  Transverse     cochlea   (Fig.    68,    Cock.)      The    ascending 
Section  through  Car-  p. 

tiiaginous  lower  part  and  descending  canals  are  separated  from 

of  Eustachian  Tube  One    another,    partly   by    a   bony   plate, 

£±L"±  ?<«%   by  a  membranous  partition-the 

and  the  way  in  which  basilar   membrane.      At  the    base,   the 

it  is  pulled  down  and  hony  iainena  js  broad,  but  at  the  apex 

the   tube    opened    in      .  ,  .  .    n 

swallowing  (shaded).      lts  place  is  chiefly  taken   by  the   mem- 
brane, which  measures  at  the  apex  more 
than  ten  times  its  width  at  the  base. 

From  the  posterior  and  superior  aspect  of  the  vestibule 
three  semicircular  canals  (Fig.  70)  open,  each  with  a 
swelling  at  one  end.  One  runs  in  the  horizontal  plane, 
and  has  the  swelling  or  ampulla  anteriorly  (Fig.  70,  h.c.). 


THE   SENSES 


133 


h.c. 


The  other  two  run  in  the  vertical  planes  indicated  in  the 
diagram,  the  anterior  being  called  the  superior  canal  (s.c.), 
and  having  its  ampulla  in  front,  the  posterior  (p.c.)  having 
its  ampulla  behind.  They  join 
together,  and  enter  the  vestibule 
by  a  common  orifice. 

The  bony  labyrinth  is  filled 
with  a  lymph-like  fluid,  the  peri- 
lymph,  and  in  it  lies  a  complex 
membranous  bag,  the  membranous 
labyrinth.  FIG>  7a_The  Keiationship  of  the 

In    the   vestibule   this   is  divided  Semicircular  Canals    to    one 

into   two   little    sacs,    the    utricle 

and  the  saccule,  joined  together 

by    a    slender    canal.     From    the 

saccule   comes   off   a    canal  which    runs   into   the   cochlea 

upon    the    basilar    membrane,   forming    a    middle   channel 

between  the  other   two,  the   scala  media   or   membranous 


p.C. 


another.  h.c.,  horizontal 
canal ;  s.  c. ,  superior  canal ; 
p.c, ,  posterior  canal. 


FIG.  71. — Transverse  Section  through  one  turn  of  the  Cochlea  to  show  the  Organ  of 
Corti  on  the  Basilar  Membrane.  S. M. ,  scala  media ;  S.V.,  scala  vestibuli ; 
S.  T. ,  scala  tympani. 

cochlea.  This  terminates  blindly  at  the  apex.  From  the 
utricle  a  membranous  canal  extends  into  each  of  the  bony 
semicircular  canals,  being  provided  with  an  ampulla,  which 


134  HUMAN  PHYSIOLOGY 

nearly  fills  up  the  bony  ampulla,  while  the  canal  portion 
is  small,  and  occupies  only  a  small  part  of  the  bony  canal. 
(Fig.  89,  p.  165.) 

In  the  membranous  cochlea  the  lining  cells  form  the  organ 
of  Gorti  (Fig.  71).  This  is  set  upon  the  basilar  membrane,  and 
consists  from  within,  outwards,  of — 1st,  A  set  of  elongated  sup- 
porting cells.  2nd,  A  row  of  columnar  cells,  with  short,  stiff, 
hair-like  processes  projecting  from  their  free  border.  3rd,  The 
inner  rods  of  Corti,  each  of  which  may  be  compared  to  an  ulnar 
bone  attached  by  its  terminal  end,  and  fitting  on  to  the  heads 
of  the  outer  rods.  4th,  The  outer  rods  of  Corti,  each  resem- 
bling a  swan's  head  and  neck — the  neck  attached  to  the  basi- 
lar membrane,  and  the  back  of  the  head  fitting  into  the  hollow 
surface  of  the  inner  rods.  5th,  Several  rows  of  outer  hair 
cells,  with  some  spindle-shaped  cells  among  them.  6th,  The 
outer  supporting  cells.  7th,  Lying  over  the  inner  and  outer 
hair  cells  is  the  membrana  reticularis,  resembling  a  net, 
through  the  meshes  of  which  the  hairs  project.  8th,  Arch- 
ing over  this  organ  is  a  homogeneous  membrane  —  the 
membrana  tectoria. 

The  membranous  labyrinth  is  attached  to  the  inner  wall 
of  the  bony  labyrinth  at  certain  points  through  which  fibres 
of  the  auditory  nerve  enter  it.  A  set  of  fibres  goes  to  the 
utricle,  a  set  to  each  of  the  ampullae,  and  a  set  to  the 
saccule  and  the  cochlea. 

The  membranous  labyrinth  has  an  outer  fibrous  coat,  and 
inside  this  a  homogeneous  layer  which  is  markedly  thick- 
ened where  the  nerves  enter  it.  It  is  lined  by  flattened 
epithelium,  which  become  columnar,  and  is  covered  with 
stiff  hair-like  processes  over  the  thickenings  at  the  entrance 
of  the  nerves. 

The  terminal  neurons  of  both  the  vestibule  and  the 
cochlea  end  in  dendrites  between  the  hair  cells,  and  the 
cell  of  each  is  upon  its  course  to  the  medulla. 

The  auditory  nerve  is  essentially  double,  consisting  of  a 
dorsal  or  cochlear,  and  a  ventral  or  vestibular  part. 

Cochlear  Root  (Fig.  72). — This  is  the  true  nerve  of  hear- 
ing. Its  fibres  (Cock.  R.)  begin  in  dendrites  between  the  cells 
of  the  organ  of  Corti,  have  a  cell  upon  their  course,  and  when 


THE   SENSES 


135 


they  enter '.the  medulla  they  branch  into  two  divisions,  which 
end  either  in  the  tuberculum  acusticum  or  the  nucleus 
accessorius  (N.  Ace.),  where  they  form  synapses.  From 
the  cells,  axons  pass  (a)  to  the  oculo-motor  mechanism 
of  the  'Same  side  and  the  opposite  side  (N.vi.\  and  (6)  up 


EYE, 


FIG.  72. — Connections  of  Cochlea  with  Central  Nervous  System.  Cock. 
R.,  cochlear  root  of  eighth  nerve  ;  N.  Ace.,  tuberculum  acusticum 
and  nucleus  accessorius  sending  fibres  to  the  cerebrum  (C.B.)  and 
to  the  oculo-motor  mechanism  (N.vi.). 

to  the   cerebrum  (CB.)  of  the   same   and   of  the  opposite 
side. 

Yestibular  Root  (Fig.  73). — The  fibres  of  this  root  take 
origin  in  dendrites  between  the  cells  of  the  maculse,  and  have 
their  nerve  cell  upon  their  course  (Ves.  R.).  As  they  enter 
the  medulla  they  divide  into  two,  forming  an  ascending 


1 36  HUMAN   PHYSIOLOGY 

and  a  descending  branch.  (1)  The  ascending  branch  sends 
fibres  on  to  the  cerebrum  (CB.\  and  to  the  superior 
vermis  of  the  cerebellum  (CBL.).  These  fibres  give  off 
collaterals  to  the  nucleus  of  Deiters  (N.  Deit.),  from 
the  cells  of  which  fibres  pass,  which  divide,  some  run- 
ning on  the  same  side,  some  on  the  opposite  side ;  one 
branch  passing  up  to  the  oculo-motor  mechanism  (N.vi.), 
the  other  passing  down  the  spinal  cord  to  send  collaterals 
to  the  cells  in  the  grey  matter.  (2)  The  descending 
branch  forms  connections  with  the  medullary  nuclei  as  it 
passes  down. 

Sound  Perception. — The  qualities  of  sound  which  can 
be  distinguished  by  the  sense  of  hearing,  are  loudness— 
amplitude  of  vibration ;  pitch — rate  of  vibration ;  and 
quality — the  character  of  the  sound  given  by  the  over- 
tones. The  perception  of  this  last  is  essentially  a  perception 
of  pitch. 

It  is  easy  to  understand  how  the  peripheral  neurons 
in  the  internal  ear  are  more  powerfully  stimulated  by 
the  greater  variations  in  the  degree  of  pressure  which 
are  produced  by  more  powerful  aerial  waves,  and  how 
the  greater  stimulation  of  the  receptive  centre  in  the 
brain  will  be  accompanied  by  a  sensation  of  greater 
loudness. 

A  study  of  the  structure  of  the  cochlea  seems  to 
show  a  mechanism  well  suited  to  afford  a  means  of 
estimating  the  pitch  of  a  note  and  the  existence  of  over- 
tones. The  fibres  of  the  basilar  membrane  may  be  com- 
pared to  the  strings  of  a  piano,  each  one  of  which,  or 
each  set  of  which,  will  be  set  in  vibration  by  a  particular 
note. 

The  power  of  distinguishing  differences  of  pitch  varies 
in  different  individuals.  It  is  more  acute  for  notes  of  a 
moderate  rate  of  vibration,  from  46  to  4000,  than  for  very 
slow  or  very  fast  vibrations. 

The  range  of  perception  of  pitch  also  varies  greatly,  some 
people  hearing  notes  as  low  as  20  vibrations  per  second, 
and  others  hearing  them  up  to  40,000  per  second. 


THE  SENSES 


137 


Y.  Taste. 

Mechanism.  —  The    nerve    endings  connected  with    the 
sense    of   taste    are   disposed   in    barrel-shaped    structures 


CELLS  OP 

ANT.  HORN 


FIG.  73. — Connections  of  Semicircular  Canals  with  Central  Nervous 
System.  Fes.  R. ,  Vestibular  root  of  eighth  nerve  sending  fibres 
upwards  to  CB.  (cerebrum)  and  CBL.  (cerebellum),  downwards  to 
the  centre  in  medulla  oblongata  (Med.)t  and  to  Deiters'  nucleus 
(N.  Deit.},  from  which  fibres  pass  to  the  oculo-motor  mechanism 
(N.vi.)  and  to  the  centres  in  the  anterior  horn  of  the  spinal 
cord. 

in  the  epithelial  covering  of  the  mouth.     These  taste  bulbs 
are  most  abundant  at  the  back  of  the  tongue,  on  the  sides 


138  HUMAN  PHYSIOLOGY 

of  the  large  circumvallate  papillae  which  form  the  prominent 
V-shaped  line  on  the  posterior  part  of  the  dorsum. 

Each  is  composed  of  a  covering  of  elongated  cells  like  the 
staves  of  a  barrel,  enclosing  a  set  of  spindle-shaped  cells  with 
which  the  dendrites  on  the  end  of  the  nerve  fibres  are  closely 
associated. 

These  nerve  fibres  pass  to  the  brain  in  the  fifth  nerve. 
Several  cases  of  complete  loss  of  taste  have  been  recorded 
by  Gowers  in  which  the  root  of  the  fifth  nerve  alone  was 
destroyed,  but  it  is  usually  thought  that  the  glosso-pharyngeal 
also  carries  nerve  fibres  connected  with  taste,  and  the  recent 
observations  favour  this  view. 

Physiology. — As  to  the  way  in  which  this  mechanism 
is  stimulated  our  knowledge  is  very  imperfect.  In  order 
to  act  the  substance  must  be  in  solution.  The  strength  of 
the  sensation  depends  on  the  concentration  of  the  solution, 
upon  the  extent  of  the  surface  of  the  tongue  acted  upon, 
upon  the  duration  of  the  action,  and  upon  the  tempera- 
ture of  the  solution.  If  the  temperature  is  very  high  or 
very  low  the  taste  sensation  is  impaired  by  the  feelings 
of  cold  or  heat. 

It  is  most  difficult  to  classify  the  many  various  taste  sensa- 
tions which  may  be  experienced,  but  they  may  roughly  be 
divided  into  four  main  groups  :— 

1.  Sweet.  3.  Acid. 

2.  Bitter.  4.  Saline. 

Whether  different  sets  of  terminations  react  specially  to 
each  of  these  is  not  known,  but  it  has  been  found  that  sub- 
stances giving  rise  to  the  sensation  which  we  call  bitter  act 
best  on  the  back  of  the  tongue,  while  substances  producing 
sweet  or  acid  sensations  act  on  the  sides  and  front.  Again, 
chewing  the  leaves  of  gymnema  sylvestre  abolishes  sensations 
of  sweet  and  bitter,  but  does  not  interfere  with  those  of  acid 
and  saline,  and  leaves  the  tactile  sense  unimpaired.  On  the 
other  hand,  cocaine  paralyses  the  tactile  sense  before  it  inter- 
feres with  the  sense  of  taste. 

The  sense  of  taste  is  very  closely  connected  with  the  sense 
of  smell,  and,  when  the  latter  is  interfered  with,  many  sub- 


THE   SENSES  139 

stances  seem  tasteless  which  under  normal  conditions  have  a 
marked  flavour. 

YI.  Smell. 

Mechanism. — Over  the  upper  part  of  the  nasal  cavity  the 
columnar  epithelial  cells  are  devoid  of  cilia,  and  between  them 
are  placed  spindle-shaped  cells  (Fig.  74,  01.  c.\  which  send 
processes  through  the  mucous  membrane,  and  through  the 
cribriform  plate  of  the  ethmoid  into  the  olfactory  bulb.  Here 


C.T*. 


FIG.  74.— The  Connections  of  the  Olfactory  Fibres.  01.  c.,  olfactory  cells  ;  01.  Gl.t 
glomeruli;  M.G.,  mitral  cells;  01.  T.,  olfactory  tract;  C.M.,  corpus 
mammilarium  ;  Hipp. ,  hippocampus  ;  Op.  Th. ,  optic  thalamus  ;  F. ,  fornix 
(modified  from  SCHAFER). 

they  form  synapses  with  other  neurons  (01.  GL),  the  axons  of 
which  pass  to  the  base  of  the  olfactory  tracts(6).  Some  fibres  (a) 
pass  straight  on  to  the  optic  thalamus,  while  others  (6)  form 
synapses  with  other  neurons,  the  fibres  of  which,  passing 
round  the  fornix,  end  by  arborising  round  cells  in  the  corpora 
mammilaria  (C.M.),  which  send  processes  on  to  the  thalamus. 
There  is  some  evidence  that  the  fibres  to  the  cortex  end  in 
the  hippocampus. 

Physiology.  —  To  act  upon  the  olfactory  mechanism  the 
substance  must  be  volatile,  and  must  be  suspended  in  the  air. 
In  this  condition  infinitesimal  quantities  of  such  substances 
as  musk  are  capable  of  producing  powerful  sensations.  The 


140  HUMAN  PHYSIOLOGY 

mucous  membrane  must  be  moist,  and  this  is  secured  by  the 
activity  of  Bowman's  glands,  situated  in  the  mucous  mem- 
brane. These  are  under  the  control  of  the  fifth  cranial  nerve, 
and  section  of  this  leads  indirectly  to  loss  of  the  sense  of 
smell. 


SECTION  V 
THE  NERVOUS  SYSTEM 

SPINAL    NERVES 

HAVING  studied  the  various  organs  of  special  sense  and  the 
muscles  by  which  we  can  react  to  the  impressions  received 
through  them,  the  general  characters  of  the  connecting 
nerves  between  the  central  nervous  system,  and  the  various 
structures  involved,  may  now  be  considered.  These  nerves 
may  be  classified  as  ingoing  and  outgoing,  and  they  may 


FIG.  75. — Structure  of  a  Typical  Spinal  Nerve.  P.R. ,  posterior  root  with  ganglion  ; 
A.R.,  anterior  root;  S.L,  ganglion  of  sympathetic  chain;  W.R.,  its  white 
ramus  ;  G.R.,  its  grey  ramus  ;  V.N.,  visceral  nerve  with  collateral  ganglion  ; 
S.N.,  somatic  nerve. 

roughly  be  divided  into  those  connected  with  the  body  wall 
and  its  appendages  and  those  connected  with  the  viscera. 

General  Structure. — The  arrangement  of  fibres  may  best 
be  understood  by  studying  the  constitution  of  one  of  the 
typical  spinal  nerves  coming  off  from,  say  the  dorsal  region, 
of  the  spinal  cord  (Fig.  75). 

A  posterior  root  (P.R.)  comes  off  from  the  postero-lateral 
aspect  of  the  cord  and  has  a  swelling  upon  it,  the  ganglion 


142  HUMAN   PHYSIOLOGY 

of  the  posterior  root.  It  joins  an  anterior  root  (A.R.) 
coming  from  the  antero-lateral  margin.  These  form  the 
spinal  nerve  which  is  distributed  to  the  body  wall.  Lying  in 
front  of  this  is  a  swelling  or  ganglion  (S.I.)  joined  to  the 
nerve  by  two  roots,  a  white  ramus  ( W.R.)  and  a  grey  ramus 
(G.R.)',  and  from  this  a  nerve  extends  towards  the  viscera 
(V.N.).  Before  this  nerve  reaches  its  final  distribution  it 
passes  through  another  ganglion. 

Roots  of  the  Spinal  Nerves. — The  posterior  root  is  the 
great  ingoing  channel  to  the  spinal  cord,  and  the  anterior 
root  is  the  great  outgoing  channel.  Section  of  a  series 
of  posterior  roots  leads  to  (a)  loss  of  sensation  in  the 
structures  from  which  the  fibres  come,  and  (b)  to  a  loss 
of  muscular  co-ordination,  as  a  result  of  cutting  off  the 
afferent  impressions  connected  with  the  muscle  sense 
(p.  97). 

As  a  result  of  this  section,  the  parts  of  the  fibres  cut  off 
from  the  cells  of  the  ganglia  on  the  posterior  root  die  and 
degenerate.  Therefore  if  the  root  is  cut  inside  the  ganglion, 
the  degeneration  extends  inwards  and  up  the  posterior 
columns  of  the  cord,  and  if  it  is  cut  outside,  the  degeneration 
passes  outwards  to  the  periphery. 

Section  of  the  anterior  root  causes  paralysis  of  the  muscles 
and  other  structures  supplied  by  the  outgoing  fibres,  and 
these  fibres  die  and  degenerate. 

The  nerve  to  the  somatopleur  or  body  wall  (S.N.)  is 
composed  of  incoming  and  outgoing  fibres.  1st,  Incoming 
fibres  are  medullated  and  take  origin  in  the  various  peri- 
pheral sense  organs.  As  they  pass  through  the  ganglion  on 
the  posterior  root  each  fibre  is  connected  by  a  side  branch 
with  a  nerve  cell — the  trophic  centre  of  the  neuron — and  it 
then  enters  the  spinal  cord,  and,  passing  to  the  postero- 
lateral  column,  it  breaks  up  into  an  ascending  branch  and  a 
descending  branch  (Fig.  40,  p.  89).  2nd,  Outgoing  fibres, 
which  are  medullated,  take  origin  from  the  large  cells  in  the 
anterior  horn  of  the  grey  matter  of  the  cord  and  pass  on  to 
be  connected  with  muscle  fibres  by  end  plates,  or  to  gland 
cells  by  less  definite  synapses. 

The  nerve  to  the  viscera  or  splanchnopleur  (V.N.),  and  to 
the  involuntary  structures  in  the  somatopleur,  contains — 1st, 


THE   NERVOUS   SYSTEM  143 

Incoming  Fibres. — These  take  origin  either  in  definite  peri- 
pheral structures,  such  as  Pacinian  corpuscles,  or  in  some 
less  defined  endings,  and  as  medullated  fibres  pass  throu^n 
the  various  ganglia,  and,  so  far  as  is  at  present  known,  have 
their  cell  stations  in  the  ganglion  on  the  posterior  root. 
2nd,  The  Outgoing  Fibres,  characterised  by  their  small  size, 
take  origin  chiefly  in  a  lateral  column  of  cells,  which  is  well 
developed  in  the  dorsal  region  of  the  cord,  and  pass  out  as 
medullated  fibres  by  the  anterior  root.  From  this  they  pass 
by  the  white  root  to  a  sympathetic  ganglion,  whence  they 
may  proceed  in  one  of  two  different  ways. 

(a)  They  may  form  synapses  with  cells,  and  from  these 
cells  fibres  may  pass — 

1.  Outwards  with  the  splanchnic  nerves ;  or, 

2.  Back  into  the  spinal  nerve  by  the  grey  root  and  so 
down  the  somatic  nerve  to  blood-vessels,  hairs,  sweat  glands, 
&c.      The  ganglia  from  which  fibres  pass  back  into  spinal 
nerve  are  known  as  lateral  ganglia. 

(b)  They  may  pass  through  these  ganglia  on  to  one  more 
peripherally  situated  in  which  they  form  synapses  and  are 
continued  onwards.     These  ganglia  from  which  fibres  do  not 
pass  back  are  called  collateral  ganglia.      Before  their  first 
interruption  they  are  termed  pre-ganglionic  fibres,  after  their 
interruption  post-ganglionic. 

The  various  fibres  after  their  interruption  proceed  as  non- 
medullated  or  grey  fibres  to  their  termination,  where  they 
break  up  into  a  network  of  anastomosing  fibres  with  cells — a 
sort  of  terminal  ganglion.  Many  drugs  have  a  special  action 
on  the  terminal  ganglia,  e.g.  apocodein  paralyses  them,  while 
adrenalin — the  extract  of  the  medullary  part  of  the  supra- 
renals — stimulates  them. 

The  interruption  of  fibres  in  ganglia,  or  their  passage 
through  these  structures,  has  been  determined  by  taking 
advantage  of  the  fact  that  nicotine  in  one  per  cent,  solution 
when  painted  on  a  ganglion  poisons  the  synapses  but  does 
not  influence  the  fibre.  Hence,  when  a  ganglion  is  painted 
with  nicotine,  if  stimulation  of  the  fibres  on  its  proximal  side 
produces  an  effect,  it  is  proved  that  the  break  is  not  in  that 
ganglion. 


144  HUMAN  PHYSIOLOGY 

General  Distribution. 

A.  SOMATIC  FIBRES. 

(a)  Outgoing  Fibres. — The  course  of  these  must  be  studied 
in  the  dissecting-room. 

(6)  Ingoing  Fibres — Cutaneous  Fibres. — The  fibres  pass- 
ing in  by  each  pair  of  nerves  come  from  zones  of  skin 
encircling  the  body.  These  are,  however,  interrupted  by  the 
limbs.  Each  limb  may  be  considered  to  be  an  outgrowth  at 
right  angles  to  the  trunk,  composed  of  a  pre-axial  and  post- 
axial  part. 

B.  SPLANCHNIC  FIBRES. 

These  are  small  medullated  fibres. 

(a)  The  Outgoing  Fibres  may  be  subdivided  as  follows 
(Fig.  76):- 

1.  Head  and  Neck. — These  leave  the  spinal  cord  by  the 
upper  five  dorsal  nerves  and  pass  upwards  in  the  sympathetic 
cord  of  the  neck  to  the  superior  cervical  ganglion  where  they 
have  their  cell  stations.     From  these,  fibres  are  distributed 
to  the  parts  supplied.     The  chief  functions  of  these  fibres 
are — 1st,  Vaso-constrictor  to  the  vessels  of  the  face  and  head  ; 
2nd,  Pupilo-dilator  (see  p.  103) ;  3rd,  Motor  to  the  muscle  of 
Miiller;    4th,  Secretory   to   the   salivary  glands,  lachrymal 
gland,  and  sweat  glands.      The  course  of  these  fibres  is  of 
importance  in  medicine,  since  tumours  in  the  upper  part  of 
the  thorax  may  press  upon  them. 

2.  Thorax. — The  fibres  to  the  thoracic  organs  also  come 
off  in  the  five  upper  dorsal  nerves,  have  their  cell  stations  in 
the  stellate  ganglion,  and  pass  to  the  heart  and  lungs. 

3.  Abdomen. — These  fibres  come  off  by  the  lower  six  dorsal 
and  upper  three  lumbar  nerves.     They  course  through  the 
lateral  ganglia  and  form  synapses  in  the  collateral  ganglia  of 
the  abdomen — the  solar  plexus  and  the  superior  and  inferior 
mesenteric  ganglia.     From  these  they  are  distributed  to  the 
abdominal  organs,  being  vaso-constrictor  to  the  vessels,  in- 
hibitory to  the  muscles  of  the  stomach  and  intestine,  and 
possibly  secretory  to  the  pancreas. 

4.  Pelvis. — The  fibres  for  the  pelvis  leave  the  cord  by  the 


THE  NERVOUS   SYSTEM 


145 


lower  dorsal  and  upper  four  lumbar  nerves,  and  have  their 
cell  stations  in  the  inferior  mesenteric  ganglia,  from  which 
they  run  in  the  hypogastric  nerves  to  the  pelvic  ganglia. 


Head. 


Thorax. 


Abdomen. 


D.I 

2. 

5 
4 
S 

6 

1 


Pelvis. 


FIG.  76. — Scheme  of  distribution  of  Splanchnic  Nerves. 


They  are  vaso-constrictor,  inhibitory  to  the  colon,  and  motor 
to  the  bladder,  uterus  and  vagina  and  the  retractor 
penis. 

5.  Arm. — These  fibres,  coming  out  by  the  fourth  to  the 
tenth  dorsal  nerves,  have  their  synapses  in  the  sympathetic 
ganglia  of  the  sympathetic  chain,  and  passing  back  into  the 

10 


146  HUMAN   PHYSIOLOGY 

spinal  nerves  by  the  grey  rami,  course  to  the  blood  vessels, 
veins,  and  sweat  glands  of  the  limb. 

6.  Leg. — The  fibres  take  origin  from  the  eleventh  dorsal 
to  the  third  lumbar  nerves,  have  their  cell  stations  in  the 
lateral  ganglia,  and  pass  to  the  leg  in  the  same  way  as  do  the 
fibres  to  the  arm. 

Outgoing  Visceral  Fibres  not  connected  with  Sympathetic 
Ganglia. — Certain  outgoing  fibres  to  the  viscera  and  involun- 
tary structures  pass  directly  without  going  through  the 
sympathetic  ganglia. 

1.  The  third  cranial  nerve  carries  fibres  which  have  their 
synapses  in  the  ciliary  ganglion,  and  pass  on  to  the  sphincter 
pupilke  and  ciliary  muscle. 

2.  The  seventh  nerve  carries  fibres   through   the   chorda 
tympani  to  cell  stations  in  the  submaxillary  and  sublingual 
ganglia.     These  are  secretory  to  the  submaxillary  and  sub- 
lingual  glands. 

3.  The  ninth  nerve  sends  fibres  to  the  parotid  gland,  which 
have  their  cell  station  in  the  otic  ganglion. 

4.  The  vagus  sends  inhibitory  fibres  to  the  heart,  which 
form  synapses  in  the  cardiac  plexus.     It  also  sends  motor 
fibres  to  the  oesophagus  and  stomach,  which,  in  some  animals 
at   least,   have    the   cell    stations   in   the  ganglion   of    the 
trunk. 

5.  The  nervi  erigentes  come  off  from  the  second  and  third 
sacral  nerves,  and  pass  to  the  hypogastric  plexus  near  the 
bladder  where  the  fibres  have  their  cell  stations.     They  are 
the  vaso-dilator   nerves   to   the  pelvic   organs,  inhibit   the 
retractor  penis,  and  are  motor  to  the  bladder,  colon,  and 
rectum. 

(6)  Ingoing  Fibres. — The  course  of  these  from  the  viscera 
is  not  so  clearly  known ;  but  they  appear  to  enter  the  main 
nerve  largely  by  the  white  rami.  In  the  normal  condition 
stimulation  of  their  peripheral  endings  does  not  lead  to 
modifications  of  consciousness,  and  is  therefore  not  accom- 
panied by  pain  (see  p.  96).  But  in  abnormal  conditions 
painful  sensations  are  produced.  In  some  cases,  abnormal 
stimulation  of  visceral  nerves  leads  to  painful  sensations 
referred  to  the  cutaneous  distribution  of  the  spinal  nerve 
with  which  they  are  connected.  Thus  disease  of  the  heart 


THE   NERVOUS  SYSTEM 


147 


is  often  accompanied  by  pain  in  the  distribution  of  the  upper 
dorsal  nerves  in  the  left  arm. 


SPINAL  CORD   AND   BRAIN. 

The  anatomy  and  histology  of  each  part  of  the  central 
nervous  system  should  be  mastered  before  its  physiology  is 


PONS. 


CORD. 


FIG.  77. — Mesial  Section  through  the  Brain  and  upper  part  of  the  Spinal  Cord 
to  show  the  positions  at  which  the  sections  figured  in  later  diagrams  have 
been  made. 


studied.     An  outline  sufficient  to  make  the  description  of 
the  physiology  intelligible  is  all  that  is  given  here. 


148 


HUMAN   PHYSIOLOGY 


A.  SPINAL   CORD. 
Structure. 

The  spinal  cord  is  a  more  or  less  cylindrical  mass  of  nerve 
tissues  which  passes  from  the  base  of  the  brain  down  the 
vertebral  canal.  It  terminates  in  a  pointed  extremity 
at  the  level  of  the  1st  lumbar  vertebra.  There  are  two 
enlargements  upon  it,  one  in  the  cervical  region,  one  in  the 
lumbar  region,  and  from  these  the  nerves  to  the  arms  and 
legs  come  off.  A  fine  central  canal  runs  down  the  middle, 


FIG.  78. — Cross  Section  of  the  Spinal  Cord  through  the  second  Dorsal  Segment, 
to  show  disposition  of  grey  and  white  matter.  P.,  Posterior  Horn ;  A.,  An- 
terior Horn  with  large  cells;  I.L.,  Intermedio-lateral  Horn  with  small  cells  ; 
L.C.,  Lockart  Clarke's  Column  of  Cells;  P.M.  and  P. L. ,  Postero-median  and 
Postero-lateral  Columns;  D.C.,  Direct  Cerebellar  Tract;  Asc.  and  Desc. 
Ant.  Lat.,  Ascending  and  Descending  Antero-lateral  Tracts;  B.B.,  Basis 
Bundles;  C.Pt/.,  Crossed  Pyramidal  Tract;  O.Py.,  Direct  Pyramidal  Tract. 
On  opposite  side  tracts  which  degenerate  upwards  are  marked  with  horizontal 
lines  ;  tracts  degenerating  downwards  with  vertical  lines.  (After  BRUCE.) 

and  the  two  sides  are  almost  completely  separated  from  one 
another  by  an  anterior  and  a  posterior  mesial  fissure  (Fig.  78). 
Each  half  is  composed  of  a  core  of  grey  matter  arranged  in 
two  processes  or  horns — the  anterior  and  posterior  horns 
(A.  and  P.) — which  divide  the  white  matter  surrounding  the 
grey  into  a  posterior,  a  lateral,  and  an  anterior  column.  In 
the  dorsal  region  a  lateral  horn  of  grey  matter  projects  into 
the  lateral  column -^(I.L.).  The  white  matter  is  composed  of 
white  nerve  fibres,  the  grey  matter  very  largely  of  cells  and 


THE  NERVOUS  SYSTEM  149 

synapses  of  neurons  supported  by  branching  neuroglia  cells. 
The  cells  of  the  grey  matter  are  largest  and  most  numerous 
in  the  anterior  horn,  where  they  constitute  the  cells  from 
which  the  majority  of  nerve  fibres  come  off.  In  the  dorsal 
region  a  group  of  cells  in  the  lateral  horn  give  off  visceral 
fibres  (I.L.).  In  the  dorsal  region  also  a  set  of  cells  lie  on 
the  mesial  aspect  of  the  posterior  horn  constituting  the  cells 
of  Lockhart  Clarke  (L.C.). 

Functions. 

The  spinal  cord  is  the  great  mechanism  of  reflex  action, 
and  the  great  channel  of  conduction  between  the  brain  and 
the  peripheral  structures. 

A.  REFLEX  FUNCTIONS. 

If  the  brain  of  such  an  animal  as  a  frog  is  destroyed,  the 
animal  lies  for  any  length  of  time  prone  on  its  belly  and 
immovable.  .If  the  skin  of  the  leg  is  pinched  the  limb  is 
withdrawn,  and  if  a  piece  of  paper  dipped  in  acetic  acid  is 
placed  on  the  flank,  definite  co-ordinated  movements  are 
made  to  remove  it.  The  animal  has  the  power  of  reflex 
movements  with  definite  co-ordination  of  its  muscles,  but  it 
has  no  power  of  balancing  itself,  and  manifests  no  spon- 
taneous movements. 

The  reflexes  conriected  with  various  groups  of  skeletal 
muscles  are  definitely  associated  with  different  levels  of  the 
cord.  In  man  the  reflex  movement  of  the  foot  on  tickling 
the  sole  is  connected  with  the  part  of  the  cord  from  which 
the  1st,  2nd,  and  3rd  sacral  nerves  come  off.  The  reflex  of 
the  cremaster  muscle  with  the  1st,  2nd,  and  3rd  lumbar; 
that  of  the  abdominal  muscles  with  the  8th  to  the  12th 
dorsal;  of  the  epigastric  muscles  with  the  4th  to  the  7th 
dorsal ;  and  of  the  scapular  muscles  with  the  5th  cervical  to 
the  12th  dorsal.  By  taking  advantage  of  these  reflexes  the 
condition  of  the  cord  at  different  levels  may  be  studied. 

Reflex  action  in  connection  with  various  visceral  muscles 
are  also  connected  with  the  spinal  cord.  Many  of  these  are 
complex  reflexes  involving  inhibition  of  certain  muscles  and 
increased  action  of  others,  some  visceral,  some  skeletal.  The 


HUMAN  PHYSIOLOGY 
best   marked   of  these   are   the   reflex   acts   of  micturition 


External  rotators  of  hij>. 
Hamstrings. 

Calf  muscles  and  extrinsic  muscles 
of  foot. 

Intrinsic  muscles  of  foot. 


Muscles  of  bladder  and  urethra. 
Extrinsic  muscles  of  foot. 
Intrinsic  muscles  of  foot. 

Levator  and  sphincter  ani. 


FIG.  79.— The  Groups  of  Cells  in  the  Anterior  Horn  of  grey  matter  at  the  level 
of  the  second,  third,  and  fourth  Sacral  Nerves.     (From  BRUCE.) 

(p.    407),  defaecation    (p.    355),    erection,    and    ejaculation 


./VVVVVA/SAAAAAAAAA/*     T. 


FIG.  80. — The  neuro-iuuscular  mechanism  concerned  in  the  knee  jerk. 
and  the  time  of  the  knee  jerk  (A.J.)  compared  with  the  time 
of  a  reflex  action  (4.72.). 


(p.  415).     The  lumbar  enlargement  is  the  part  of  the  cord 
involved. 


THE  NERVOUS   SYSTEM  151 

The  synapses  in  the  cord  are  not  only  capable  of  acting 
reflexly  to  set  up  definite  contractions  in  muscles,  but  they 
seem  to  exercise  a  constant  tonic  action  upon  them,  and 
when  this  tonic  action  is  interfered  with,  the  effect  of  directly 
stimulating  a  muscle  is  diminished.  This  is  very  well  seen 
in  the  contraction  of  the  quadriceps  extensor  femoris  which 
occurs  when  the  ligamentum  patellae  is  struck  sharply  causing 
a  kick  at  the  knee  joint — the  knee  jerk  (Fig.  80).  When 
the  reflex  arc  in  the  cord  in  the  lower  lumbar  region  is 
interfered  with,  the  knee  jerk  is  diminished  or  is  absent,  and 
when  the  activity  of  the  arc  is  increased  by  the  removal  of 
the  influence  of  the  brain,  the  jerk  is  increased.  That  the 
jerk  is  not  a  reflex  is  shown  by  the  fact  that  the  latent 
period  is  very  much  shorter  than  that  of  a  reflex  action 
(Fig.  80).  The  reflex  arc,  however,  is  necessary  for  the  tonus. 
This  tonus  is  increased  by  tension  of  the  muscle  and  also  by 
fatigue  of  the  nervous  system  which  may  lead  to  cramp. 

The  degeneration  of  the  cells  which  follow  amputation  of 
the  leg  at  different  levels  seems  to  indicate  that  the  various 
groups  of  cells  in  the  anterior  horn  of  grey  matter  have 
definite  connections  with  individual  muscles  (see  Fig.  79). 


B.  CONDUCTING  PATHS. 

Outgoing  and  ingoing  fibres  chiefly  pass  down  the  side  of 
the  cord  upon  which  they  act  or  from  which  they  come. 
Section  of  one  side  of  the  cord  leads  to  loss  of  the  so-called 


„  

/ 

.-_„_-„-      -/, 

77 

/,!— 

t 

/// 

Spinal  Nerve 

FIG.  81.— To  show  the  course  of  upgoing  (dotted  line)  and  downgoing 
fibres  (continuous  line)  in  the  spinal  cord. 

voluntary  movements  and  loss  of  sensation  below  the  point 
of  section  on  the  same  side.  Since  there  is,  at  least  some- 
times, a  slight  loss  of  voluntary  power  and  of  sensation  on 


152  HUMAN  PHYSIOLOGY 

the  opposite  side,  it  has  been  concluded  that  a  few  of  both 
sets  of  fibres  decussate  in  the  cord.  It  will  afterwards  be 
shown  that  the  main  set  of  fibres  cross  the  middle  line  in 
the  medulla  (Fig.  83). 

The  two  kinds  of  fibres  run  in  different  strands  or  tracts 
of  the  cord,  and  these  tracts  have  been  defined  by  different 
methods. 

1.  Degeneration  or  Wallerian  Method. — This  depends 
upon  the  fact  that  nerve  fibres  degenerate  when  cut  off 
from  their  cells  (p.  86),  and  that  they,  generally  speaking, 
conduct  in  the  direction  in  which  they  degenerate,  although, 
as  has  been  seen  in  the  fibres  of  the  posterior  roots  of  spinal 
nerves,  this  is  not  always  the  case.  If  the  spinal  cord  be  cut 
across,  certain  tracts  of  fibres  degenerate  upwards,  others 
degenerate  downwards,  while  some  do  not  degenerate  to  any 
great  distance  from  the  point  of  section. 

Degenerations  which  reveal  these  tracts  are  often  produced 
by  diseases  and  injuries  of  the  cord,  and  thus  the  results 
experimentally  produced  on  animals  have  been  confirmed  on 
the  human  subject.  These  degenerations  may  be  demon- 
strated when  recent  by  Marchi's  method  of  staining,  which 
depends  upon  the  fact  that  while  the  white  sheath  of  normal 
fibres  is  not  stained  black  when  the  tissue  is  placed  in  a 
solution  of  chrome  salt  with  osmic  acid,  it  is  so  stained  when 
it  begins  to  degenerate  (p.  86).  When  the  white  sheaths 
have  entirely  disappeared  the  degeneration  is  best  demon- 
strated by  Weigert's  method  of  staining  the  white  sheaths  of 
normal  fibres  with  hsematoxylin,  which  leaves  the  degenerated 
fibres  unstained. 

A.  Fibres  degenerating  upwards  (see  Fig.  78,  p.  148).— 
1.  The  fibres  making  up  the  posterior  columns  which  are 
derived  from  the  posterior  roots  of  the  spinal  nerves  (P.L. 
and  P.M.).  A  few  of  these  fibres  degenerate  downwards, 
because  the  fibres  of  the  posterior  roots  bifurcate  when  they 
enter  the  cord  (Fig.  40,  p.  89),  the  one  division  passing  right 
up  to  the  top  of  the  cord,  the  other  passing  down  for  a  short 
distance.  The  ascending  fibres  of  the  posterior  columns  for 
the  most  part  end  in  synapses  in  the  upper  end  of  the  cord, 
in  the  nucleus  gracilis  and  nucleus  cuneatus  (Fig.  40,  p.  89). 
From  these  fresh  fibres  pass  upwards  to  the  cerebrum 


THE  NERVOUS  SYSTEM  153 

(Fig.  40,  (7.).  2.  A  thin  layer  of  fibres  round  the  margin  of  the 
lateral  column  degenerates  upwards,  and  has  been  traced  as 
far  as  the  cerebellum  (Fig.  78,  D.C. ;  Fig.  40,  E.).  It 
consists  of  two  sets  of  fibres.  Those  behind  constitute  the 
direct  or  dorsal  cerebellar  tract ;  those  in  front  which  take 
a  somewhat  different  course  forming  the  ascending  antero- 
lateral  or  ventral  cerebellar  tract  (Fig.  78,  Asc.  Ant.  Lat.). 
They  both  take  origin  in  Lockhart  Clarke's  column  of  cells, 
which  is  specially  well  developed  in  the  dorsal  and  upper 
lumbar  region  of  the  cord. 

B.  Fibres    degenerating    downwards. — 1.  A  very  strong 
band   of  fibres   lying   in  the   posterior  part  of  the  lateral 
column,  just  inside  the  direct  cerebellar  tract,  and  becoming 
smaller  as  the  lower  part  of  the  cord  is  reached.     This  is 
the  crossed  pyramidal  tract  (Fig.  78,  C.Py.),  which  comes 
from  the  cells  of  the  cortex  cerebri  of  the  opposite  side,  and 
gives  off  collateral  branches  to  the  cells  in  the  anterior  horn 
of  the  spinal  cord  (Fig.  40,  D.). 

2.  Certain  fibres  from  the  cortex  cerebri  do  not  cross,  but 
run  down,  some  in  the  crossed  pyramidal  tract,  some  in  the 
direct  pyramidal  tract  (Fig.  78,  O.Py.),  which  runs  along  the 
margin  of  the  anterior  fissure,  and  extends  downwards  only 
into  the  dorsal  region.     These  fibres  decussate  in  the  cord. 

3.  A  set  of  fibres  just  inside  the  antero-lateral  ascending 
tract,  which   may  be   called   the  antero-lateral  descending 
tract  (Fig.  78,  Desc.  Ant.  Lat.).     This  comes  from  Deiters' 
nucleus  (see  p.  89),  and,  as  it  passes  down,  gives  off  fibres  to 
the  cells  in  the  anterior  horn  of  the  grey  matter  of  the  cord. 
Deiters'  nucleus  receives  fibres  from   the  cerebellum,  and 
these  fibres  probably  carry  down  impulses  from  that  organ. 

C.  Fibres  not  degenerating  for  any  distance. — Round  the 
grey  matter,  a  band  of  fibres — the  basis  bundles  (Fig.  78, 
B.B.) — do  not  degenerate  far,  and  seem  to  be  commissural 
between  adjacent  parts  of  the  grey  matter. 

Other  tracts  of  fibres  have  been  described,  such  as  Lis- 
sauer's  tract  and  the  septo-marginal  tract,  but  their  relations 
have  not  been  satisfactorily  investigated. 

2.  Developmental  Method. — The  development  of  the  cord 
also  helps  to  demonstrate  the  various  tracts,  since  it  has  been 
found  that  the  fibres  of  outgoing  tracts  become  functionally 


154 


HUMAN   PHYSIOLOGY 


active,  and  get  their  medullary  sheath  later  than  the  fibres 
of  ingoing  tracts.  The  crossed  and  direct  pyramidal  tracts 
are  non-medullated  at  birth,  while  the  ingoing  tracts  are 
medullated. 


nc- 


cyen 


-pern. 


Fio.  82. — View  from  above  of  the  medulla  oblongata,  corpora  quadri- 
gemina,  and  optic  thalami.  c.l.a.,  posterior  columns  of  cord  ; 
VIII.,  XII.,  JT.,  indicate  the  nuclei  of  these  cranial  nerves 
in  the  floor  of  the  fourth  ventricle  ;  p.c.i. ,  the  restiform  body  ; 
p.c.m.,  the  middle  peduncle  of  the  cerebellum;  p.c.s.,  the 
superior  peduncle  of  the  cerebellum;  t.q.,  the  anterior  and 
posterior  corpora  quadrigemina ;  c.o. ,  the  optic  thalamus  with 
pulvinar  (pulv.)  and  external  and  internal  geniculate  bodies 
behind  it ;  e.p.,  the  pineal  body.  (VAN  GEHUCHTEN.) 


B.  THE   MEDULLA  OBLONGATA. 
1.  Structure. 

The  medulla  oblongata  may  be  regarded  as  the  upper  end 
of  the  spinal  cord,  and  it  connects  that  structure  with  the 
brain  (Fig.  82).  The  cord  expands  and  the  posterior  mesial 


THE  NERVOUS   SYSTEM  155 

fissure  is  opened  out,  so  that  the  central  canal  comes  to  the 
surface,  and  expands  into  a  lozenge-shaped  area — the  floor 
of  the  fourth  ventricle.  The  lateral  columns  of  the  cord 
pass  outwards  to  the  cerebellum  to  form  part  of  its  inferior 
peduncles — the  restiform  bodies.  Between  the  lateral  and 
the  anterior  columns  an  almond-shaped  swelling,  the  olive, 
appears  (Fig.  84,  0.).  Above  this  the  medulla  is  encircled 
by  a  mass  of  transverse  fibres  —  the  middle  peduncles  of 
the  cerebellum,  or  the  pons  Varolii  (Fig.  86,  P.).  The 
floor  of  the  fourth  ventricle  is  constricted  above  by  the 
approximation  of  the  superior  peduncles  of  the  cerebellum 
to  again  become  a  canal. 

The  grey  matter  of  the  cord  gets  broken  up  into  separate 
masses,  of  which  the  most  important  are  : — 

1.  The  nuclei  of  the  posterior  columns — the  nucleus  gracilis 
and  nucleus  cuneatus  (Fig.  83,  N.C.  and  N.G.) — masses  of 
cells   and   synapses    in  which   the   fibres  of    the   posterior 
columns  end,  and  from  which  the  upgoing  fibres  of  the  fillet 
start. 

2.  The  inferior  olivary  nucleus  (Fig.  84,  0.),  which  lies  in 
the  olive,  and  which  is  connected  by  bands  of  fibres  with  the 
dentate  nucleus  of  the  cerebellum  (Fig.  86,  Deit.). 

3.  The  nucleus  of  Deiters  (Fig.    86,   Deit.),  lying  higher 
up   in   the   pons  Varolii,  and   connected   with   fibres  from 
the    cerebellum    and    from    the    semicircular    canals    (see 
Fig.  73). 

4.  The  masses  of  cells  from  which  the  cranial  nerves  take 
origin  (Fig.  85). 

2.  Conducting  Paths. 

A.  Ingoing. — 1.  The  posterior  columns  of  the  spinal  cord 
terminate  in  two  masses  of  grey  matter  on  each  side,  the 
nucleus  gracilis  and  nucleus  cuneatus.  From  these,  fibres 
pass  downwards  and  across  the  middle  line  forming  the 
decussation  of  the  fillet  (Fig.  83,  F.).  The  crossed  fibres 
(Fig.  84,  F.)  then  pass  up  in  a  vertical  series  on  each  side 
of  the  middle  line  until  the  pons  Varolii  is  reached,  when 
they  spread  out  horizontally  like  a  fan  (Fig.  86,  F.)  above 
the  deep  transverse  fibres.  Above  the  pons  they  divide 
into  two  sets  (Fig.  90,  F.) — a  lateral  fillet,  which  ends  in 


156 


HUMAN  PHYSIOLOGY 


the  anterior  corpora  quadrigemina,  and  a  mesial  fillet,  which 
passes  on  to  the  optic  thalamus,  and  there  ends  by  forming 
synapses. 

2.  The  direct  cerebellar  tract  passes  up  into  the  restiform 
body,  and  so  on  to  the  superior  vermis  of  the  cerebellum.     Its 
fibres  form  synapses  round  cells  chiefly  on  the  opposite  side. 

3.  The  ascending  antero-lateral   tract  passes   up   beside 
the  last,  but  it  leaves  it  in  the  restiform,  and  courses  forward, 
to  arch  back  into  the  cerebellum  round  the  superior  cere- 


FIG.  83. — Cross  Section  through  Medulla  Oblongata  above  the  decussation  of  the 
Pyramids.  P.M.  and,  P.L.,  Postero-median  and  Postero-lateral  tracts  of  the 
Cord  ;  N.G.  and  N.C.,  Nucleus  Gracilis  and  Cuneatus.  giving  off  the  Fillet 
Fibres  crossing  at  F.  ;  V.,  Ascending  Root  of  Fifth  Nerve;  G.t  Nucleus  of 
Glossopharyngeal  Nerve  ;  A.H.,  Anterior  Horn  of  Spinal  Cord  ;  P.,  The 
Anterior  Pyramids;  Z>.(7.,  Direct  Cerebellar  Tract;  A.  and  D.  Ant.  L., 
Ascending  and  Descending  Antero-lateral  Tracts.  (After  BRUCE.) 

bellar  peduncle  and  to  form  synapses  with  the  cells  of  the 
superior  vermis. 

B.  Outgoing. — 1.  The  fibres  from  the  cerebral  cortex, 
which  form  in  the  cord  the  crossed  and  direct  pyramidal 
tracts,  pass  down  in  the  middle  part  of  the  crusta  (Fig. 
90,  P.)  of  the  crura  cerebri,  and  coursing  between  the  super- 
ficial and  deep  transverse  fibres  of  the  pons  (Fig.  86,  P.), 
come  to  lie  in  the  anterior  pyramids  of  the  medulla  (Fig. 
83,  P.).  At  the  lower  end  of  the  medulla  most  of  these 
fibres  cross  over  to  the  lateral  column  of  the  cord;  some, 


THE  NERVOUS  SYSTEM 


157 


however,  run  down  the  direct  and  crossed  pyramidal  tracts 
of  the  same  side. 

2.  The  fibres  of  the  descending  antero -lateral  tract  take 
origin  in  a  mass  of  nerve  cells,  which  lies  in  the  dorsal  and 
lateral  part  of  the  pons  Varolii  (Deiters'  nucleus)  (Fig.  86, 
Deit.). 

C.  Commissural  Fibres. 

(1)  The  antero-lateral  basis  bundles  of  the  cord  form  in 
the  medulla  a  strong  band  of  fibres  connecting  the  grey 


V7/7  AccN 


FIG.  84. — Cross  Section  of  Medulla  through  the  Olive.  The  Central  Canal  has 
opened  out  to  form  the  Floor  of  the  Fourth  Ventrical,  4th  V.  ;  the  Lateral 
Columns  are  passing  out  to  form  the  Inferior  Peduncles  of  the  Cerebellum  ; 
F. ,  Fillet;  O. ,  Inferior  Olivary  Nucleus;  />.,  Anterior  Pyramids;  Rest., 
Fibres  of  Kestiform  Body;  F.,  Ascending  Root  of  Fifth  Nerve;  VIII. 
Ace.  N.,  Accessory  Nucleus  of  the  Eighth  Nerve.  (After  BRUCE.) 

matter  at  different  levels,  and  known  as  the  posterior  longi- 
tudinal fasciculus. 

(2)  A  set  of  fibres  run  from  each  olivary  body  across  the 
middle  line  to  the  dentate  nucleus  of  the  cerebellum  of  the 
opposite  side. 

3.  Cranial  Nerves. 

(The  physiology  of  these  should  be  studied  while  dissecting 
them.)  The  nerves  springing  from  and  entering  the  medulla, 
do  not  come  off  in  the  same  regular  fashion  as  the  spinal 
nerves.  The  outgoing  fibres  of  each  spring  from  a  more  or 


158  HUMAN   PHYSIOLOGY 

less  definite  mass  of  cells.  The  ingoing  fibres  generally  form 
synapses  with  cells  arranged  in  definite  groups.  In  this  way 
the  so-called  nuclei  of  the  cranial  nerves  are  formed.  The 
position  of  these  is  indicated  in  Fig.  85.  In  the  cranial 
nerves  no  sharp  differentiation  into  anterior  and  posterior 
roots  can  be  made  out,  but  they  contain  the  same  component 
elements  as  the  spinal  nerves,  the  fibres  running  either 
together  or  separately. 

Ingoing  Fibres. — Somatic  and  splanchnic  enter  the  medulla 
and  have  their  cell  stations  in  ganglia  upon  the  nerves. 

Outgoing  Fibres. — Somatic  and  splanchnic  pass  out,  the 


FIG.  85.— The  Nuclei  and  Roots  of  the  Cranial  Nerves.     (After  EDINGER.) 

latter  being  characterised  by  their  small  size,  and  by  forming 
synapses  before  their  final  distribution. 

The  XII.  (Hypoglossus)  is  purely  an  anterior  root  nerve, 
and  is  motor  to  the  muscles  of  the  tongue. 

The  X.  (Vagus)  is  essentially  the  posterior  root  of  the  XI. 
(Spinal  Accessory),  but  it  transmits  some  outgoing  fibres. 
It  is  the  great  ingoing  nerve  from  the  abdomen,  thorax, 
larynx,  and  gullet,  while,  by  outgoing  fibres  passing  through 
the  vagus  or  accessorius,  it  is  augmentor  for  the  muscles  of 
the  bronchi  and  alimentary  canal,  inhibitory  to  the  heart, 
dilator  to  blood-vessels  of  the  thorax  and  abdomen,  and 
motor  to  the  muscles  of  the  larynx  and  to  the  levator  palati. 
The  accessorius  is  also  motor  to  the  sterno-cleido-mastoid 
and  trapezius. 

The  IX.  (Glossopharyngeal)  is  essentially  a  posterior  root, 
and  is  the  ingoing  nerve  for  the  back  of  the  mouth,  the 
Eustachian  tube,  and  tympanic  cavity.  It  transmits  out- 


THE  NERVOUS  SYSTEM 


159 


going  fibres  which  are  motor  to  the  stylo-pharyngeus  and 
middle  constrictor  of  the  pharynx. 

The  III.  (Oculomotorius),  IV.  (Trochlearis),  VI.  (Ab- 
ducens),  and  VII.  (Facial),  along  with  the  V.  (Trigeminal), 
form  what  may  be  regarded  as  a  pair. 

The  Trigeminal  is  chiefly  a  posterior  root,  but  it  has  a 


Rest 


FlG.  86. — Cross  Section  through  Region  of  Pons,  Cerebellum,  and  Fourth  Ventricle. 
S.  V. ,  Superior  Vermis ;  M.N.,  Roof  Nucleus;  Dent.,  Dentate  Nucleus; 
Rest.,  Restiform  Body;  S.P.,  Superior  Peduncle  of  Cerebellum;  Deit., 
Deiters'  Nucleus;  VI.,  Nucleus  of  the  Sixth  Nerve;  F.,  Fillet;  S.O., 
Superior  Olive  ;  D.  T.  and  S.T.,  Deep  and  Superficial  Transverse  Fibres  ;  P., 
Pyramidal  Fibres.  (After  BRUCE.  ) 


distinct  anterior  or  motor  root  which  joins  it,  and  carries  the 
motor  fibres  to  the  muscles  of  mastication. 

It  is  the  great  ingoing  nerve  for  all  the  face. 

The  VII.  is  almost  purely  an  anterior  root,  transmitting 
the  motor  fibres  to  the  muscles  of  expression,  and  secretory 
fibres  to  the  submaxillary  and  sublingual  glands  and  the 


i6o  HUMAN  PHYSIOLOGY 

glands  of  the  mouth.  It,  however,  carries  ingoing  fibres  from 
the  anterior  two- thirds  of  the  tongue. 

The  VI.  supplies  the  external  rectus  of  the  eye. 

The  IV.  supplies  the  superior  oblique.  The  III.  supplies 
all  the  muscles  of  the  eyes  except  those  supplied  by  the  VI. 
and  IV. 

The  fibres  coming  from  the  nuclei  of  these  cranial  nerves 
do  not  always  pass  out  in  the  nerve  itself.  Thus,  fibres  from 
the  nucleus  of  the  III.  to  the  orbicularis  oculi  pass  out  in 
the  VIL,  while  fibres  for  the  posterior  belly  of  the  digastric 
which  pass  out  in  the  VII.  probably  come  from  the  nucleus 
of  the  XII. 

4.  Reflexes  of  the  Medulla. 

The  extensive  series  of  synapses  in  the  medulla  form 
arrangements  by  which  various  combined  and  co-ordinated 
movements  are  controlled.  Thus,  part  of  the  nucleus  of  the 
vagus  governs  the  movements  of  respiration,  while  other 
parts  preside  over  the  slowing  mechanism  of  the  heart.  To 
these  various  reflex  arrangements  the  name  of  centres  has 
been  given. 

C.  REGION  OF  PONS  VAROLII. 

Outgoing  Fibres. — 1.  The  fibres  to  the  face  muscles  cross 
the  middle  line  to  become  associated  with  the  various  nuclei 
of  the  cranial  nerves.  For  this  reason  a  tumour  in  one  side 
of  the  pons  may  cause  paralysis  of  the  face  muscles  on  one 
side  and  of  the  muscles  of  the  rest  of  the  body  on  the 
opposite  side.  2.  Fibres  to  the  limbs  and  trunk  run  down 
between  the  deep  and  superficial  transverse  fibres  (Fig. 
86,  P.). 

Ingoing  Fibres. — The  fillet  fibres  in  the  pons,  instead  of 
running  up  on  each  side  of  the  middle  line,  spread  out  into  a 
horizontal  arrangement  above  the  crossed  fibres. 


D.  CEREBELLUM. 

1.  Structure. — The  cerebellum  or  lesser  brain  lies  above 
the  fourth  ventricle,  and  is  joined  to  the  cerebro-spinal  axis 
by  three  peduncles  on  each  side.  It  consists  of  a  central 


THE   NERVOUS  SYSTEM 


161 


lobe,  the  upper  part  of  which  is  the  superior  vermis,  and  two 
lateral  lobes,  each  with  a  secondary  small  lobe,  the  flocculus. 
Its  surface  is  raised  into  long  ridge-like  folds  running  in  the 
horizontal  plane,  and  is  covered  over  with  grey  matter,  the 
cortex.  In  the  substance  of  the  white  matter  forming  the 
centre  of  the  organ  are  two  masses  of  grey  matter  on  each 
side — 1,  the  roof  nucleus;  and  2,  the  dentate  nucleus 
(Fig.  86,  R.N.  and  Dent.). 

The  cortex  may  be  divided  into  an  outer  somewhat  homo- 
geneous layer  (the  molecular  layer,  Fig.  87,  G.L.)  and  an  inner 


FIG.  87.— Diagram  of  the  Arrangement  of  Fibres  and  Cells  in  the  Cortex  of  the 
Cerebellum.  G.L.,  Molecular  Layer;  N.L.,  Nuclear  Layer;  P.,  Purkinje's 
Cells.  (After  RAMON  Y  CAJAL.  ) 

layer  studded  with  cells  (the  nuclear  layer,  N.L.).  Between 
these  is  a  layer  of  large  cells — the  cells  of  Purkinje  (P.). 

By  Golgi's  method  the  arrangement  of  fibres  and  cells  in 
the  cerebellum  has  been  shown  to  be  as  follows : — 

Fibres  coming  into  the  cortex  from  the  white  matter  end 
either  in  synapses  round  cells  in  the  nuclear  layer,  or  proceed 
at  once  to  the  outer  layer  (Fig.  87).  From  the  cells  in  the 
nuclear  layer  processes  pass  to  the  outer  layer  and  there  form 
synapses  with  other  cells.  From  these,  processes  pass  to 
the  cells  of  Purkinje,  round  which  they  arborise,  and  from 

11 


162  HUMAN  PHYSIOLOGY 

Purkinje's  cells  the  outgoing  fibres  of  the  cerebellum  pass 
into  the  white  matter,  to  the  roof  nuclei,  and  hence  to 
Deiters'  nuclei  (Fig.  86). 

2.  Connections.  —  The    cerebellum    is    connected    (Fig. 
88):— 

i.  With  the  Spinal  Cord. 

a.  Incoming  Fibres. — 1.  The  direct  cerebellar  tract  (p.  156) 
passes  up  in  the  restiforin  body  to  end  chiefly  in  the  superior 
vermis.  2.  The  ascending  antero-lateral  tract  (p.  156)  passes 
to  the  cerebellum  in  the  superior  peduncle  and  ends  in 
the  superior  vermis.  3.  Fibres  from  the  nuclei  of  the 
posterior  columns  of  the  same  side  (Fig.  83,  p.  156)  pass 
in  the  restiform  body  to  the  cerebellum.  4.  Fibres  from 
the  vestibular  root  of  the  eighth  nerve  also  pass  to  the 
cerebellum  (p.  137). 

6.  Commissural  Fibres. — Strong  bands  of  fibres  connect 
the  inferior  olive  of  one  side  with  the  dentate  nucleus  of 
the  other. 

c.  Outgoing  Fibres. — Fibres  pass  from  the  superior  vermis 
to  the  roof  nuclei,  and,  from  these,  fibres  pass  on  to  Deiters' 
nuclei  (Fig.  86,  p.  159),  from  which  fibres  pass  down  in  the 
descending  antero-lateral  tract  of  the  cord. 

ii.  With  the  Cerebrum. — 1.  The  fibres  of  the  middle 
peduncles  cross  in  the  middle  line  embracing  the  medulla, 
and  become  associated  with  cells  from  which  fibres  pass  up 
in  the  lateral  parts  of  the  crura  cerebri  to  the  cerebral 
cortex  (Fig.  90,  CO.  CO.,  p.  167).  2.  The  fibres  of  the  superior 
peduncle  cross  and  end  in  the  red  nuclei  (Fig.  90,  S.C.P.), 
from  which  fibres  seem  to  pass  upwards  to  the  cerebrum. 
How  far  these  are  upward  conducting  and  how  far  down- 
ward is  not  definitely  known. 

3.  Functions. — Removal  of  the  cerebellum  deprives  the 
animal,  for  a  time  at  least,  of  the  power  of  balancing  itself. 
This  may  be  easily  demonstrated  in   the  pigeon  (Fig.   95, 
p.  171).     But  in  some  cases,  when  slowly  progressing  disease 
has  destroyed  the  organ,  no  loss  of  equilibration  has  appeared, 
and  in  other   cases  the   cerebellum  has  been  congenitally 
almost  absent,  and  yet  the  individual  has  not  shown  any 
sign   of  want  of  power  of  maintaining   his  balance.      Evi- 


THE  NERVOUS   SYSTEM 


163 


dently,  therefore,  some  other  part  of  the  brain  can  com- 
pensate for  its  absence. 

The  manner  in  which  the  cerebellum  acts  has  been  chiefly 
elucidated  by  removing  parts  of  the  organ  and  keeping  the 


FIG.  88. — Connections  of  the  Cerebellum  with  the  Cerebro-spinal  Axis  (for 
explanation,  see  text). 


animals  under  observation  for  prolonged  periods.  If  one 
side  of  the  cerebellum  is  removed  the  first  symptoms  are 
(1)  a  tonic  contraction  of  the  muscles  of  the  limbs  of  the 
same  side  by  which  the  fore  limbs  may  be  powerfully  ex- 


1 64  HUMAN   PHYSIOLOGY 

tended,  and  an  arching  of  the  body  with  the  convexity 
towards  the  side  of  the  lesion,  while  the  animal  may  be 
driven  round  its  long  axis  to  the  opposite  side.  (2)  These 
irritative  symptoms  soon  pass  off,  and  the  animal  then  mani- 
fests inadequacy  or  weakness  in  the  limbs  of  the  affected 
side,  so  that  it  droops  to  that  side,  and  if  a  quadruped  may 
circle  to  that  side.  (3)  After  some  weeks  these  symptoms 
disappear,  and  the  loss  of  one  side  of  the  cerebellum  is 
apparently  completely  compensated  for. 

From  these  experiments  it  would  appear  that  the  cere- 
bellum is  to  be  regarded  as  a  mechanism  supplementary  to 
the  great  cerebro-spinal  mechanism,  and  that  it  has  for  its 
purpose  more  especially  the  muscular  adjustment  required 
in  maintaining  the  balance.  This  it  may  do  in  one  or  both 
of  two  ways. 

1.  By  receiving    impulses    from    without,    and    sending 
impulses  downwards  to  act  upon  the  spinal  mechanism. 

2.  By  receiving  impulses,  and  sending  impulses  upwards 
to  the  cerebrum  to  modify  its  action.     A  channel  for  such 
impulses  exists  in  the  fibres  of  the  pons  which  cross  the 
middle  line  to  connect  with  cells  from  which  fibres  pass 
upwards  to  the  occipital  and  frontal  lobes  of  the  cerebrum 
(Fig.  40,  p.  89). 

To  maintain  the  constant  muscular  adjustments  involved 
in  balancing  the  body  requires  an  arrangement  whereby  any 
disturbance  of  the  equilibrium  can  produce  an  appropriate 
reaction. 

The  ingoing  impulses  which  are  more  especially  of  service 
in  this  way  are  (1)  the  muscular  sense  (see  p.  97);  (2)  the 
tactile  sense  from  the  soles  of  the  feet ;  (3)  vision ;  and  (4) 
the  sense  of  acceleration  or  retardation  of  motion  in  any 
plane  or  planes  of  the  body  derived  through  the  semicircular 
canals.  The  importance  of  the  muscular,  tactile,  and  visual 
senses  in  maintaining  the  balance  is  so  obvious  that  it  need 
not  be  further  considered. 

4.  Physiology  of  the  Semicircular  Canals. — That  there  is 
no  special  mechanism  making  us  aware  of  uniform  movement 
is  proved  by  the  fact  that  we  are  not  conscious  of  whirling 
through  space  with  the  earth's  surface,  and  that  in  a  smoothly 


THE  NERVOUS   SYSTEM  165 

running  train  we  lose  all  sense  of  forward  movement.  It  is 
only  as  the  train  starts  or  stops  that  we  have  a  sensation  of 
movement  or  retardation.  The  same  thing  has  been  demon- 
strated by  strapping  a  man  to  a  table  rotating  smoothly 
round  a  vertical  axis  and  setting  the  table  spinning.  A 
sense  of  rotation  is  experienced  as  the  table  starts  but  is  lost 
when  the  movement  becomes  uniform,  while  stopping  the 
table  gives  rise  to  a  sensation  of  being  rotated  in  the  opposite 
direction. 

The  semicircular  canals  form  a  mechanism  which  is 
capable  of  acting  in  this  way.  They  are  arranged  in  pairs 
in  the  two  ears  thus — The  two  horizontal  canals  are  in  a 
horizontal  plane,  the  superior  canal  of  one  side  and  the 
posterior  canal  of  the  other  are  in  parallel  planes  oblique 
to  the  mesial  plane  of  the  body  (Fig.  89,  a). 


FIG.  89.— (a)  Arrangement  of  the  Semicircular  Canals  on  the  two  sides ;  (6)  Bony 
and  Membranous  Canal  and  Ampulla  to  illustrate  their  mode  of  action. 

The  horizontal  canals  may  be  considered  as  forming  the 
arc  of  a  circle  with  an  ampulla  at  each  end.  The  superior 
canal  of  one  side  has  its  ampulla  in  front,  while  its  twin — the 
posterior  of  the  opposite  side — has  its  ampulla  behind,  and 
they  together  form  the  arc  of  a  circle  with  an  ampulla  at 
each  end  (Fig.  89,  a). 

The  membranous  canals  are  very  narrow,  and  occupy  but 
a  small  part  of  the  osseous  canals.  The  membranous 
ampullae  are  large  and  almost  fill  the  osseous  ampullae 
(Fig.  89,  6). 

If  the  head  is  moved  in  any  plane,  certain  changes  will  be 
set  up  in  the  arnpullse  towards  which  the  head  is  moving, 
and  converse  changes  in  the  ampullae  at  the  other  end  of  the 
arc  of  the  circle. 


1 66  HUMAN  PHYSIOLOGY 

If,  for  example,  the  head  is  suddenly  turned  to  the  right, 
the  inertia  of  the  endolymph  and  perilymph  tend  to  make 
them  lag  behind.  Thus  the  endolymph  in  the  ampulla  of 
the  right  horizontal  canal  will  tend  to  flow  into  the  canal, 
but  the  canal  is  so  small  that  it  will  merely  accumulate  in 
the  ampulla,  and  thus  a  high  pressure  will  be  produced  (Fig. 
89,  b  +  +).  The  perilymph  will  tend  to  lag  behind,  and  a 
low  pressure  will  result  outside  (Fig.  89,  b  —  ).  The  converse 
will  take  place  in  the  opposite  horizontal  canal. 

When  the  movement  is  continued  the  pressures  will  be 
readjusted,  and,  on  stopping  the  movement,  the  opposite 
conditions  will  be  induced,  and  a  sensation  of  moving  in  an 
opposite  direction  will  be  experienced. 

In  forward  movement,  the  two  superior  canals  have  the 
pressure  of  endolymph  increased  in  their  ampullae — in  back- 
ward movement  this  occurs  in  the  two  posterior  canals.  In 
nodding  to  the  right  the  superior  and  posterior  canals  of  the 
right  ear  undergo  this  change. 

There  is  also  evidence  that  the  semicircular  canals  assist 
in  maintaining  the  tone  of  the  skeletal  muscles  and  that 
destruction  of  the  canals  is  followed  by  a  loss  of  tone.  This 
might  be  expected  from  their  intimate  connection  with  the 
cerebellum. 

When  the  information  as  to  our  relationship  with  our 
surroundings  derived  from  these  various  sources  is  not 
concordant — e.g.  when  through  the  semicircular  canals  we 
have  a  sensation  of  movement,  and  through  the  eyes  an 
apparent  absence  of  movement — balancing  becomes  difficult, 
and  a  feeling  of  giddiness  results.  This  may  be  readily 
demonstrated  by  setting  a  poker  vertically  on  the  floor, 
holding  it  in  the  hand,  placing  the  forehead  on  the  top, 
walking  rapidly  three  times  round  it,  then  standing  up  and 
trying  to  walk  out  of  the  room.  The  sudden  stoppage  of 
the  rotatory  movement  causes  a  disturbance  in  the  semi- 
circular canals  giving  a  sense  of  rotation  in  the  opposite 
direction,  while  the  eyes  tell  us  that  no  rotation  is  taking 
place.  The  feeling  of  giddiness  is,  however,  not  the  cause 
of  the  loss  of  balancing,  but  a  mere  accompaniment. 
(Experiment.) 


THE  NERVOUS   SYSTEM 


167 


E.  THE   CRURA  CEREBRI   AND   CORPORA 
QUADRIGEMINA. 

Above  the  pons  Varolii,  the  two  halves  of  the  medulla 
diverge  from  one  another  and  form  the  peduncles  of  the 
cerebrum  (Fig.  90,  CC.,  P.),  while  posteriorly  the  two  superior 
peduncles  of  the  cerebellum  having  crossed  join  together 
(8.C.P.).  Above  these,  two  swellings  develop  on  each  side — 
the  anterior  and  posterior  corpora  quadrigemina  (Fig.  82, 
p.  154). 

The  crusta,  or  anterior  parts   of  each   peduncle   of  the 


FIG.  90. — Cross  Section  through  Anterior  Corpora  Quadrigemina  and  Cerebral 
Peduncles,  A.S.,  Aqueduct  of  Sylvius;  III.,  Nucleus  of  Third  Nerve; 
S.C.P.,  Superior  Cerebellar  Peduncles;  F.,  Fillet;  P.,  Pyramidal  Tract; 
CC.,  Cerebello-cerebral  Fibres.  (After  BRUCE.) 

cerebrum,  is  composed,  in  its  central  part,  of  the  pyramidal 
fibres  passing  down  from  the  cerebrum  to  the  spinal  cord 
(P.),  and,  on  each  side,  of  the  cerebello-cerebral  fibres  pass- 
ing upwards  from  the  pons  (CC.).  The  posterior  part,  or 
tegmentum,  contains — 1st,  the  fillet  fibres  going  partly  to 
the  corpora  quadrigemina,  partly  onwards  to  the  thalamus 
opticus  (P.);  2nd,  the  nuclei  of  the  3rd  and  4th  cranial 
nerves;  3rd,  the  fibres  of  the  superior  peduncles  of  the 
cerebellum  which  cross  the  middle  line  (S.C.P.);  and  4th, 
the  red  nuclei  in  which  most  of  these  fibres  end. 


168 


HUMAN  PHYSIOLOGY 


The  functions  of  this  segment  of  the  brain  are  chiefly 
conducting,  but  the  anterior  corpora  quadrigemina  forms  the 
shunting  station  between  the  incoming  fibres  of  the  optic 
tract  and  the  oculo-motor  mechanism  (see  p.  126). 


FlG.  91. — Diagrammatic  Horizontal  Section  through  Base  of  Cerebral  Hemisphere, 
showing  (1)  the  Outgoing  Fibres  for  the  Leg,  Arm,  and  Face  springing  from 
the  Cortex  of  the  Rolandic  Areas,  passing  through  the  Internal  Capsule 
between  the  Thalamus  and  the  Lenticular  Nucleus.  The  Face  Fibres  cross  in 
the  Pons,  the  Leg  and  Arm  Fibres  in  the  Medulla.  (2)  The  Incoming  Fibres 
(Fillet,  Eye)  form  their  stations  in  the  Thalamus,  and  then  pass  on  to  the 
Cortex. 


F.  THE  CEREBRUM. 

Each  peduncle  terminates  in  its  half  of  the  cerebrum. 
As  the  fibres  pass  from  peduncle  to  cerebrum  and  vice  versa 
they  come  into  relationship  with  three  masses  of  grey  matter 
lying  in  the  midst  of  the  cerebrum.  These  are  the  thalamus 
options  into  which  the  ingoing  fibres  enter;  the  lenticular 
nucleus,  between  which  and  the  thalamus  the  outgoing 


THE  NERVOUS   SYSTEM 


169 


fibres  run ;  and  the  caudate  nucleus,  the  main  part  of  which 
lies  in  front  of  the  other  two  (Fig.  91). 


FIG.  92. — Diagram  of  the  Arrangement  of  Cells  in  a  typical  part  of  the  Cerebral 
Cortex  (see  p.  168).     (After  RAMON  Y  CAJAL.) 

The  fibres,  above  these  nuclei,  spread  out   to   form   the 


FIG.  93. — Diagram  of  Collateral  Connections  of  different  parts  of  the  Cerebral 
Cortex,  a,  &,  c,  Pyramidal  Cells  of  the  Cortex,  all  connected  by  Collateral 
Branches  with  other  parts  of  the  Cortex  in  the  same  and  in  the  opposite 
hemisphere,  a  gives  off  the  Pyramidal  Fibres  to  the  Cord.  (After  RAMON  Y 
CAJAL.) 

corona  radiata  and  enter  a  crust  of  grey  matter,  the  cortex 
cerebri,  which  covers  over  the  cerebrum,  and  which  in  the 


HUMAN  PHYSIOLOGY 

higher  animals  is  raised  into  a  number  of  folds  or  con- 
volutions marked  off  from  one  another  by  fissures  and  sulci 
(Fig.  94). 

The  method  of  Golgi  shows  that  the  neurons  in  a  typical 
part  of  the  cortex  are  arranged  in  the  following  manner 
(Fig.  92) :— 


FiQ.  94. — A  shows  the  chief  convolutions  on  the  outer  aspect  (front  to  right),  and 
B  on  the  inner  aspect  (front  to  left)  of  a  cerebral  hemisphere. 

1.  The  incoming  fibres  pass  right  up  to  the  surface  and 
end  in  dendrites  (1). 

2.  In  this  region  are  a  set  of  small  horizontal  cells  with 
numerous  processes  (2). 

3.  Underneath  these  are  several  rows  of  small  pyramidal 
cells  sending  dendritic  processes  to  the  surface  and  their 
axons  downwards  (3). 

4.  Below  these  are  rows  of  large  pyramidal  cells  similarly 
arranged  (4). 


THE  NERVOUS   SYSTEM 


171 


5.  Lowest  of  all  are  some  irregular  cells  with  a  similar 
disposition  of  processes.  (These  are  not  shown  in  Fig. 
92.) 

This  arrangement  is  considerably  modified  in  certain 
regions  of  the  cortex,  the  large  pyramidal  cells  being  best 
developed  in  those  parts  from  which  the  great  mass  of  fibres 
pass  down  to  the  spinal  cord. 

From  the  various  fibres  collaterals  come  off  which  connect 
different  parts  of  the  cortex  of  the  same  side,  and  which  also 
connect  the  cortex  of  one  side  with  that  of  the  other,  and 
with  the  basal  ganglia  (Fig.  93). 

The  diagram  opposite  shows  the  more  important  con- 
volutions of  the  cortex  cerebri  (Fig.  94). 


FIG.  95. — A,  Pigeon  with  the  Cerebellum  destroyed  to  show  struggle  to  maintain 
the  balance ;  JB,  Pigeon  with  Cerebrum  removed  to  show  balance  maintained, 
but  the  animal  reduced  to  a  somnolent  condition. 

Functions  of  the  Cerebrum — 1.  General  Consideration.— 

The  functions  of  the  cerebrum  may  be  best  understood  by 
first  contrasting  the  condition  of  animals  with,  and  of  animals 
without,  this  part  of  the  brain. 

(1)  In  the  frog  the  cerebral  lobes  may  easily  be  removed. 
The  animal  sits  in  its  characteristic  attitude.    When  touched 
it  jumps,  when  thrown  into  water  it  swims.     It  is  a  perfect 
reflex  machine,  with  the  power  of  balancing  itself  unimpaired. 
But   it   differs   from  a  normal   frog  in  moving  only  when 
directly  stimulated,  and  in  showing  no  signs  of  hunger  or  of 
thirst.     A  worm  crawling  in  front  of  it  does  not  cause  the 
characteristic  series  of  movements  for  its  capture  which  are 
seen  in  a  normal  frog. 

(2)  In   the  pigeon  (Fig.  95,  B),  removal  of  the  cerebral 
hemispheres  reduces   the   animal    to    the    condition    of    a 


i/2  HUMAN   PHYSIOLOGY 

somnolent  reflex  machine.  The  bird  sits  on  its  perch, 
generally  with  its  head  turned  back,  as  if  sleeping.  If  a 
sudden  noise  is  made,  if  light  is  flashed  in  its  eye,  or  if  it  is 
touched,  it  flies  off  its  perch  and  lights  somewhere  else. 
But  every  stimulus  produces  the  same  result.  Clapping 
the  hands  and  letting  peas  fall  on  the  floor  are  both 
obviously  heard  and  both  produce  a  start,  but  the  bird 
makes  no  endeavour  to  secure  the  peas  as  it  would  do  in  the 
normal  state. 

(3)  In  the  dog,  by  a  succession  of  operations,  Goltz  has 
removed   the  greater  part  of  the  cerebral   cortex   without 
causing  paralysis  of  the  muscles.     The  animal  became  dull 
and  listless,  and  did  not  take  food  unless  it  was  given  to  it. 
It  showed  no  sign  of  recognising  persons  or  other  dogs,  and 
did  not  respond  in  the  usual  way  when  patted  or  spoken  to. 
But  it  snapped  when  pinched,  shut  its  eyes  and  turned  its 
head  away  from  a  bright  light,  and  shook  its  ears  at  a  loud 
sound.     It  did  not  sit  still,  but  walked  constantly  to  and  fro 
when  awake.    It  slept  very  heavily.    In  fact  all  the  responses 
of  the  animal  might  be  classed  as  reflex  responses  to  imme- 
diate excitation. 

(4)  In  monkeys,  removal  of  the  cerebral  cortex  leads  to 
such   loss  of  the   so-called  voluntary  movements   that   all 
other  symptoms  are  masked. 

In  the  decerebrated  animal  different  stimuli  do  not  produce 
distinctive  reactions,  but,  with  the  cerebrum  intact,  there  is 
at  least  the  possibility  of  small  differences  in  the  modes  of 
stimulation  producing  marked  differences  in  the  resulting 
action. 

These  resulting  actions  are  in  part  at  least  determined  by 
(1)  the  previous  training  and  education  of  the  brain;  for, 
just  as  in  the  spinal  cord  channels  of  action  are  formed,  so 
in  the  cerebrum,  if  a  given  reaction  once  follows  a  given 
stimulus,  it  will  tend  to  follow  it  again,  (a)  This  training  is 
in  part  hereditary.  Each  individual  of  a  race  is  born  with 
well-established  lines  of  action  in  the  process  of  development, 
and  throughout  life  these  inherited  channels  play  an  im- 
portant part  in  determining  the  results  of  stimulation. 
(b)  But  it  is  also  largely  acquired  by  the  individual,  since 
the  reception  of  each  stimulus  and  the  performance  of  a 


THE   NERVOUS  SYSTEM 


173 


resulting  action,  however  this  be  determined,  tends  to  lay 
down  a  path  which  will  again  be  followed. 

(2)  Not  only  will  the  previous  training  of  the  brain  thus 
act  as  the  directive  force  in  the  response  to  stimuli,  but  the 
nutrition  of  the  brain  also  plays  an  important  part.  The 


S  TOMNG. 


RECEIVING. 


\ 


p  I  (VA  L      AR.C 


FIG.  96. — Diagram  to  illustrate  different  possible  channels  of  cerebral 
response  to  stimulation. 


action  of  a  brain  when  well  nourished  and  freely  supplied 
with  pure  blood  is  often  very  different  to  that  of  the  same 
brain  when  badly  nourished  or  imperfectly  supplied  with 
healthy  blood.  Since  the  education  of  the  brain  really  con- 
sists in  developing  proper  responses  to  various  stimuli,  the 


174  HUMAN   PHYSIOLOGY 

importance  of  the  brain  being  in  a  healthy  and  well-nourished 
condition  during  the  training  is  manifest. 

The  power  of  differentiating  various  stimuli  is  dependent 
on  the  development  of  the  brain,  and  becomes  more  perfect 
as  the  animal  scale  is  ascended ;  and  the  complexity  of  the 
cerebral  action  in  the  higher  animals  has  its  basis  in  the 
greater  number  of  distinct  impressions  which  have  been 
received  and  reacted  to.  Each  separate  stimulus  leaves  its 
mark  upon  the  brain,  or  as  we  may  say,  is  sto?*ed  in  the  brain, 
and  each  subsequent  similar  stimulus  is  sent  into  these 
channels,  or  is  associated  with  the  past  reactions,  and  thus 
the  present  response  is  determined,  what  may  be  described 
as  an  unconsciousness  judgment  being  made.  For  appro- 
priate reaction  the  whole  mechanism  must  be  normal — a 
very  small  injury  to  any  part  may  completely  alter  the 
character  of  the  response  to  any  given  stimulus — as  may  be 
well  seen  in  the  insane. 

So  far,  cerebral  action  may  be  considered  in  a  purely 
material  manner  as  consisting  of  a  series  of  reflex  acts,  higher 
and  more  complex  than  those  in  the  spinal  cord,  in  which 
the  result  of  the  stimulus  varies  in  the  same  way,  but  to  a 
much  greater  degree,  from  its  association  with  more  complex 
past  impressions. 

But  this  cerebral  action  is  generally  accompanied  by 
changes  in  the  consciousness  of  the  individual,  some  of 
which  are  termed  simply  sensations,  while  others  may  be 
described  as  trains  of  thought.  Essentially,  however,  a  train 
of  thought  is  nothing  more  than  a  train  of  sensations,  each 
evoked  by  a  stimulus  or  by  a  preceding  sensation,  and,  if 
this  be  admitted,  we  may  say  that  it  is  by  sensation  alone 
that  we  are  aware  of  consciousness,  and  that,  therefore,  the 
two  are  coterminous.  It  is  impossible  to  conceive  conscious- 
ness without  sensation,  or  sensation  without  consciousness. 

Cerebral  action  frequently  goes  on  without  consciousness 
being  implicated ;  but  so  far  as  we  know,  consciousness 
without  accompanying  cerebral  action  is  unknown,  and  there 
is  evidence  that  it  is  only  when  the  action  of  the  various 
parts  of  the  cerebrum  is  co-ordinated  that  consciousness 
is  possible.  In  cases  of  Jacksonian  epilepsy,  as  a  result 
of  a  small  centre  of  irritation  on  the  surface  of  the  brain, 


THE  NERVOUS  SYSTEM  175 

a  violently  excessive  action  of  the  cerebral  neurons  starts 
at  the  part  irritated  and  passes  to  involve  more  and  more 
of  the  brain.  In  such  fits  it  is  found  that  at  first  the 
patient's  consciousness  is  not  lost,  but  that,  when  a  suffi- 
cient area  of  brain  is  involved  in  this  excessive  and  unco- 
ordinated action,  consciousness  disappears. 

The  study  of  the  action  of  drugs  which  abolish,  conscious- 
ness— e.g.  chloroform  and  morphine — on  the  dendrites  of 
brain  cells,  suggests  a  physical  explanation  of  the  loss  of 
consciousness.  It  is  found  that  these  drugs  cause  a  general 
extension  of  the  gemmules  of  all  the  dendrites ;  and,  if  we 
imagine  that  the  co-ordinated  action  of  any  part  of  the  brain 
is  secured  by  definite  dendrites  of  one  set  of  neurons  coming 
into  relationship  with  definite  dendrites  of  another  set  of 
neurons  by  their  gemmules,  the  want  of  co-ordinate  relation- 
ship established  by  the  general  expansion  might  obviously 
explain  the  disappearance  of  the  definite  sensations  which 
constitute  consciousness. 

It  is  manifest  that  the  range  of  consciousness  must  neces- 
sarily be  wider  where  the  stored  impressions  are  most 
abundant,  and  where  the  present  stimulus  most  readily 
calls  into  action  these  previous  lines  of  cerebral  activity.  The 
storage  of  impressions  is  the  basis  of  MEMOKY,  the  power  of 
associating  these  stored  impressions  with  the  present  stimu- 
lus is  the  basis  of  RECOLLECTION.  It  is  the  implication 
of  consciousness  in  this  part  of  brain  action  which  is  the 
basis  of  mental  activity.  How  far  the  mental  action  is  a 
mere  accompaniment  of  the  physical  changes,  and  how 
far  it  can  react  upon  them,  is  a  question  which  cannot  be 
discussed  here.  But  the  study  of  the  insane  seems  to  point 
to  the  conclusion  that  the  individual  does  certain  things  and 
has  certain  ideas  as  concomitant  results  of  faulty  brain 
action,  rather  than  that  his  actions  are  a  result  of  the 
modified  ideas. 

2.  Time  of  Cerebral  Action — The  cerebral  mechanism 
takes  a  very  appreciable  time  to  act,  and  the  time  varies 
with  the  complexity  of  the  action  and  with  the  condition 
of  the  nervous  apparatus. 

Of  the  time  between  the  presentation  of  a  flash  of  light  to 


i/6  HUMAN  PHYSIOLOGY 

the  eye  or  a  touch  to  the  skin  and  a  signal  made  by  the 
person  acted  upon  when  it  is  perceived,  part  is  occupied 
by  the  passage  of  the  nerve  impulses  up  and  down  the 
nerves  and  in  the  latent  period  of  muscle  action,  but  some- 
thing over  TVth  of  a  second  remains,  representing  the  time 
occupied  in  the  cerebral  action.  (Practical  Physiology, 
Chap.  XII.) 

If  the  observation  be  complicated  by  requiring  a  dis- 
crimination to  be  made  between  different  stimuli,  the  reaction 
time  is  longer,  and,  the  more  unaccustomed  the  differentia- 
tion, the  longer  will  the  reaction  time  be.  Thus  in  one 
accustomed  to  deal  with  figures,  the  discrimination  of  a 
series  of  these  is  more  rapidly  carried  out  than  in  one 
unaccustomed  to  do  so.  Prolonged  action  of  the  nerve 
centres  soon  leads  to  a  prolongation  of  the  reaction  time, 
and  the  same  thing  is  produced  by  the  action  of  alcohol, 
chloroform,  and  other  poisons.  This  fatigue  of  the  nerve 
mechanism  is  the  physical  basis  of  that  state  of  the  con- 
sciousness which  is  called  loss  of  attention. 

3.  Fatigue  of  Cerebral  Mechanism. — This  naturally  leads 
to  the  consideration  of  fatigue  of  the  cerebral  mechanism. 
The  way  in  which,  as  a  result  of  poisons,  the  definite 
co-relationship  of  certain  sets  of  neurons  with  certain  other 
sets  is  abolished  by  the  generalised  expansion  of  the 
gemmules  of  the  dendrites  has  been  already  dealt  with. 
In  all  probability  the  same  thing  occurs  in  fatigue,  and  by 
interrupting  the  definite  chain  of  action  allows  rest  and 
recovery  to  supervene.  But  continued  action  leads  to  well- 
marked  changes  in  the  cell  protoplasm  of  the  neurons. 
The  Nissl's  granules  diminish  and  the  nucleus  shrivels  and 
becomes  poorer  in  chroinatin. 

In  all  reflex  action,  whether  spinal  or  cerebral,  it  is  the 
central  part  of  the  mechanism  which  first  becomes  fatigued. 
If,  by  direct  excitation  of  the  central  nervous  system  of 
a  frog,  muscular  movements  are  caused  for  some  time,  the 
stimulation  ultimately  fails  to  act ;  but,  if  the  nerves  going 
to  the  muscles  are  stimulated,  the  muscles  at  once  respond, 
showing  that  the  central  mechanism  has  given  out  before 
the  peripheral  structures. 


THE  NERVOUS  SYSTEM  177 

Fatigue  of  the  central  nervous  system  is  manifest  both 
upon  the  receiving  and  reacting  mechanism ;  upon  the 
receiving,  on  the  physical  side  by  prolongation  of  the 
reaction  period,  and  on  the  metaphysical  side  by  diminished 
power  of  attention.  Upon  the  reacting  side  it  is  shown  by 
lessened  power  of  muscular  contraction.  (See  also  p.  94.) 

4.  Sleep. — Fatigue  of  the  cerebral  mechanism  is  closely 
connected  with  sleep.  As  the  result  of  fatigue,  external 
stimuli  produce  less  and  less  effect,  and  thus  the  changes 
which  are  the  physical  basis  of  consciousness  become  less 
and  less  marked.  At  the  same  time,  by  artificial  means 
stimuli  are  usually  excluded  as  far  as  possible.  Absence  of 
light,  of  noise,  and  of  tactile  and  thermal  stimuli  all  conduce 
to  sleep.  Consciousness  fades  away,  and,  as  the  cerebral 
activity  diminishes,  the  arterioles  throughout  the  body 
dilate,  and  the  arterial  blood  pressure  falls,  and  thus  less 
blood  is  sent  to  the  brain,  and  the  organ  becomes  more 
bloodless.  The  eyelids  close,  the  eyeballs  turn  upwards, 
the  pupils  contract,  and  the  voluntary  muscles  relax. 

The  depth  of  sleep  may  be  measured  by  the  strength  of 
the  stimuli  required  to  overcome  it,  and  it  has  been  found 
that  usually  it  is  deepest  at  about  the  end  of  an  hour, 
and  that  it  then  rapidly  becomes  more  and  more  shallow 
until,  as  the  result  of  some  stimulus,  or  when  the  brain  has 
regained  its  normal  condition,  it  terminates.  In  the  later 
hours  of  sleep  the  consciousness  may  be  temporarily  aroused 
without  the  other  conditions  of  sleep  disappearing,  and  as  a 
result  of  this  dreams  may  ensue.  Or,  on  the  other  hand, 
without  consciousness  being  necessarily  restored,  stimuli  may 
lead  to  muscular  response  of  a  perfectly  definite  and  pur- 
posive character,  and  sleep-walking  may  occur. 

Hypnosis  is  a  condition  in  some  respects  allied  to  sleep. 
It  may  be  produced  in  many  individuals  by  powerfully 
arresting  the  attention,  and  is  probably  due  to  a  removal 
of  the  influence  of  the  higher  centres  over  the  lower.  When 
the  condition  is  produced  the  respiration  and  pulse  become 
quickened,  the  pupil  expands,  the  sensitiveness  of  the  neuro- 
muscular  mechanism  is  so  increased  that  merely  stroking 
a  group  of  muscles  may  throw  them  into  firm  contraction. 

12 


1 78  HUMAN   PHYSIOLOGY 

The  individual  becomes  a  pure  reflex  machine  even  as 
regards  the  cerebral  arc,  and  each  stimulus  is  followed 
by  an  immediate  reaction.  The  power  of  suggestion  is 
exaggerated.  If  a  hypnotised  person  is  told  that  he  sees 
anything  he  acts  at  once  as  if  he  did  actually  see  it. 

5.  Localisation  of  Functions. — The  question  must  n«  xt, 
be  considered  whether  special  parts  of  the  brain  are  more 
especially  connected  with  its  three  great  functions— 

1.  The  reception  of  stimuli. 

2.  The  storing  of  effects,  and  the  associating  of  present 
stimuli  with  these  stored  impressions. 

3.  The  production  of  the  resulting  actions. 

1.  Reception  of  Stimuli. — In  investigating  the  existence  of 
special  mechanisms,  for  this  purpose,  two  methods  of  inquiry 
are  available. 

1st.  By  removing  or  stimulating  parts  of  the  brain  in  the 
lower  animals  and  studying  the  results. 

2nd.  By  observations  during  life  on  the  sensations  or 
absence  of  sensation  in  patients  suffering  from  disease  of 
the  brain,  and  the  determination  of  the  seat  of  the  lesion 
after  death. 

1st.  Sensations  are  the  usual  accompaniment  of  the  activity 
of  the  receiving  mechanism.  But,  in  the  lower  animals,  it  is 
not  possible  to  have  a  direct  expression  of  whether  sensations 
are  experienced  or  not,  and,  therefore,  in  determining  whether 
removal  of  any  part  of  the  brain  has  taken  away  the  power 
of  receiving  impressions,  we  have  to  depend  on  the  absence 
of  the  usual  modes  of  response  to  any  given  stimulus.  But 
the  absence  of  the  usual  response  may  mean,  not  that  the 
receiving  mechanism  is  destroyed,  but  either  that  the  react- 
ing mechanism  is  out  of  action  or  that  the  channels  of 
conduction  have  been  interfered  with.  (See  Fig.  97.) 

Thus,  if  light  be  flashed  in  the  eye  of  a  monkey,  it  re- 
sponds by  glancing  towards  the  source  of  illumination :  and 
if  these  movements  are  absent  this  may  be  due  to  loss  of  the 
receiving  mechanism,  to  loss  of  the  mechanism  causing  the 
movements,  or  to  interruption  of  the  channels  between  these. 


THE  NERVOUS  SYSTEM 


179 


Again,  it  is  quite  possible  that,  after  removing  the  receiving 
mechanism  in  the  cerebrum,  external  stimuli  may  lead  to 
the  usual  response  by  acting  through  lower  reflex  arcs 
(Fig.  97).  Thus,  if  we  suppose  the  receiving  part  of 
the  cerebrum  connected  with  the  reception  of  tactile  im- 


RECCIVING. 


» 


D14CHAP.C.ING. 


FlG.  97. — Diagram  to  show  how  through  reflex  action  of  the  lower  arcs  the  action 
of  the  higher  arcs  may  be  stimulated. 

pressions  entirely  destroyed,  scratching  the  sole  of  the  foot 
may  still  cause  the  leg  to  be  drawn  up,  just  as  if  a  sensation 
had  been  experienced.  Here  although  the  upper  arc  is  out 
of  action  the  lower  arc  still  acts. 

In  the  lower  animals,  stimulation  of  a  part  of  the  brain, 
if  it  be  connected  with  the  reception  of  impressions,  may 
cause  the  series  of  movements  which  naturally  follow  such 
an  impression.  But  these  movements  may  also  be  caused 

B 

Of     CM, 


i8o 


HUMAN   PHYSIOLOGY 


by  directly  stimulating  the  reacting  mechanism.  When, 
however,  removal  of  a  part  of  the  brain  causes  no  loss  of 
power  of  movement,  and  }^et  prevents  a  stimulus  from 
causing  its  natural  response,  it  is  justifiable  to  conclude  that 
that  part  of  the  brain  is  connected  with  reception. 


Km.  98.— (a)  Surface  of  the  left  Cerebral  Hemisphere  to  show  the  situations  of 
some  of  the  Receiving  and  Discharging  Mechanisms  (front  to  left) ;  (b)  Mesial 
Surface  of  the  same  Hemisphere  (front  to  right). 

2nd.  In  man,  the  chief  difficulty  of  obtaining  information 
is  in  finding  cases  where  only  a  limited  part  of  the  brain  is 
affected.  But  such  cases  have,  in  many  instances,  been 
observed.  Tumours  of  an  occipital  lobe,  for  instance,  have 
been  found  to  be  associated  with  loss  of  visual  sensations 


THE   NERVOUS  SYSTEM  181 

without  loss  of  muscular  power,  and  thus  the  conclusion  has 
been  drawn  that  the  occipital  lobe  is  the  receiving  mechanism 
for  stimuli  from  the  eyes. 

Visual  Centre. — The  way  in  which  the  fibres,  coming  from 
the  two  retinae,  are  connected  with  each  thalamus  opticus  and 
occipital  lobe  has  been  already  considered,  and  it  has  been 
shown  that  the  optic  tract  passes  into  the  geniculate  bodies 
on  the  posterior  aspect  of  the  thalamus,  and  that  a  strong  band 
of  fibres,  called  the  optic  radiation,  extends  from  these  back- 
wards to  the  occipital  lobes  (Fig.  66,  p.  127).  An  extensive 
lesion  of  one — say  the  right — occipital  lobe,  especially  if  on 
the  inner  aspect  in  the  region  of  the  cuneate  lobe,  is  accom- 
panied by  no  loss  of  muscular  power  but  by  blindness  for  all 
objects  in  the  opposite  side  of  the  field  of  vision — i.e.  the  right 
side  of  each  retina  is  blind.  The  central  spot  of  neither  eye 
is  completely  blinded  because  the  fibres  from  the  macula 
lutea  only  partially  decussate  at  the  chiasma.  Probably  each 
part  of  the  occipital  lobe  is  connected  with  definite  parts  of 
the  two  retinse.  Certain  it  is  that  there  is  no  part  of  this 
cortical  mechanism  connected  with  the  blind  spot,  and  hence 
this  is  not  perceived  in  ordinary  vision. 

Colour  perception  seems  to  be  a  less  fundamental  function 
of  the  visual  apparatus  than  perception  of  the  degree  and 
direction  of  illumination,  and  hence  colour  perception  may  be 
lost  in  less  extensive  lesions  of  the  occipital  lobes  without  the 
other  functions  being  impaired.  One  of  the  most  recently 
acquired  functions  of  the  mechanism  is  the  power  of  appre- 
ciating the  significance  of  the  signs  used  in  written  language, 
and  it  is  found  that  in  small  and  superficial  lesions  this 
function  may  alone  be  lost. 

The  cortex  of  the  occipital  lobe  is  rich  in  the  smaller  variety 
of  cells  and  poor  in  the  larger  pyramidal  cells,  which  are  found 
in  areas  connected  with  the  production  of  movements. 

While  there  is  good  evidence  that  a  special  localised  area 
of  the  cortex  is  connected  with  the  reception  of  stimuli  from 
the  eyes,  the  evidence  of  the  existence  of  similar  areas  or 
centres  connected  with  the  other  organs  of  special  sense  is 
by  no  means  satisfactory. 

Auditory    Centre. — Ferrier,    by    removing     the    superior 


1 82  HUMAN   PHYSIOLOGY 

temporo-sphenoidal  lobe  in  the  monkey,  produced  no  motor 
disturbance,  but  found  evidence  of  loss  of  hearing  in  the 
opposite  ear.  He  described  the  monkey  as  pricking  its  ears, 
and  looking  to  the  opposite  side  when  the  region  is  stimu- 
lated, and  he  considered  that  these  observations  prove  the 
existence  of  a  special  localised  mechanism  for  the  reception 
of  stimuli  from  the  ear.  But,  more  recently,  Schafer  has 
removed  these  convolutions  from  both  sides  in  the  monkey, 
with  aseptic  precautions,  kept  the  animal  alive,  and  found 
no  evidence  of  loss  of  auditory  sensations.  This  conclusion 
depends  upon  the  observation  that  stimulation  of  the  auditory 
mechanism  in  such  animals  still  leads  to  the  usual  muscular 
response.  But  since  lower  connections  probably  exist  between 
the  auditory  nuclei  in  the  medulla  and  the  centres  for  mus- 
cular movement  in  the  spinal  cord  (Figs.  72  and  73),  this 
observation  cannot  be  accepted  as  excluding  the  relationship 
of  the  superior  temporo-sphenoidal  convolution  with  hearing. 
This  relationship  is  strongly  supported  by  pathological  evi- 
dence. Cases  of  "  Jacksonian  Epilepsy"  have  been  recorded, 
in  which  the  first  symptom  of  the  fit  was  the  hearing  of 
sounds,  and  in  which  the  lesion  was  found  on  this  lobe. 

Taste  and  Smell. — So  far  satisfactory  evidence  of  the  exist- 
ence of  a  special  localised  mechanism  connected  with  these 
senses  is  wanting.  Ferrier  states  that  removal  of  the  hippo- 
campal  convolution  in  monkeys  leads  to  loss  of  taste  and 
smell,  and  that  stimulation  causes  torsion  of  the  nostrils  and 
lips,  as  if  sensations  of  smell  and  taste  were  being  experienced. 
But  Schafer  was  unable  to  observe  any  indication  of  loss 
of  taste  and  smell  in  monkeys  from  which  the  temporo- 
sphenoidal  lobe  had  been  to  a  large  extent  removed. 

Other  experimenters  have  observed  interference  with  the 
sense  of  smell  in  destructive  lesions  of  the  hippocampal  lobe, 
and  one  case  at  least  has  been  described  in  which  a  tumour 
of  the  right  gyrus  hippocampus  was  associated  with  sensa- 
tions of  smell.  In  monotremes  the  fibres  from  the  olfactory 
lobe  have  been  traced  to  the  neighbourhood  of  the  hippo- 
campus. 

Touch. — Here,  again,  the  difficulty  of  drawing  conclusions 
from  observations  on  lower  animals  is  encountered.  Ferrier 
thought  that  removal  of  the  hippocampal  convolution  caused 


THE  NERVOUS  SYSTEM  183 

loss  of  tactile  sense,  while  Schafer  believes  that  the  gyrus  for- 
nicatus  is  the  centre  for  this  sense.  It  has  been  objected 
that  in  removing  this  lobe  the  fibres  going  to  the  areas  on 
the  outside  of  the  cortex  are  apt  to  be  injured.  According 
to  the  observations  of  Mott,  when  the  cortex  round  the  fissure 
of  Rolando — in  which  the  mechanism  for  causing  the  various 
combinations  of  muscular  movements  is  situated — is  removed 
in  the  monkey,  clips  may  be  attached  to  the  skin  on  the 
opposite  side  of  the  body  without  attracting  attention, 
while  if  they  are  placed  on  the  same  side  they  are  at  once 
removed.  He  therefore  regards  the  Rolandic  area  of  the 
brain  as  connected  with  the  reception  of  tactile  impressions. 

As  already  indicated,  this  centre  must  act  as  a  chart  of  the 
surface  of  the  body,  stimulation  of  any  definite  part  of  the 
body  leading  to  changes  in  a  definite  part  of  the  centre,  and 
these  changes  are  accompanied  by  sensations  referred  to  the 
part  stimulated. 

We  know  nothing  of  the  existence  of  centres  connected 
with  the  thermal  and  muscular  senses. 

2.  Storing  and  Association  Mechanism. — The  existence  of 
a  special  part  or  parts  of  the  brain  connected  with  the  storing 
of  impressions,  so  that  they  may  be  associated  with  present 
sensations,  is  indicated  by  the  following  considerations : — 
It  is  this  association  of  present  stimuli  with  past  sensations 
which  is  the  basis  of  intellectual  life,  and  in  man,  where 
apparently  the  intellectual  functions  are  most  highly  deve- 
loped, the  frontal  lobes  of  the  brain  are  much  larger  than  in 
the  lower  animals.     So  far  stimulation  of  these  frontal  lobes 
has  failed  to  give  indication  of  resulting  sensations  or  to 
produce  muscular  movements.     They  may  be   extensively 
injured  without  loss  of  sensation  and  without  paralysis,  and 
hence  it  has  been  concluded  that  in  them  the  storing  and 
associating  functions  must  be  chiefly  located. 

3.  Discharging    Mechanism. — The   position   of   the   dis- 
charging mechanism  for  cerebral  action  has  been  definitely 
localised,  by  pathological  and  experimental  observation,  in 
the  cerebral  convolutions  round,  or  probably  chiefly  in  front  of, 
the  fissure  of  Rolando  or  sulcus  centralis.    Destructive  lesions 


1 84 


HUMAN   PHYSIOLOGY 


of  this  area  on  one  side  cause  a  loss  of  the  so-called  voluntary 
action  of  groups  of  muscles  on  the  opposite  side  of  the  body. 
The  cerebral  arc  is  stimulated  and  acts  along  certain  lines — 
possibly  with  the  accompaniment  of  consciousness  in  a  sen- 
sation of  decision  as  to  the  line  of  action  to  be  taken  and  a 
desire  to  accomplish  it — but  this  so-called  volition  is  not 
accompanied  by  the  appropriate  muscular  action.  From 
the  frequent  involvement  of  the  so-called  volition  in  these 
actions,  and  from  the  fact  that  these  metaphysical  changes 


Fio.  99. — Left  Hemisphere  of  Brain  of  Chimpanzee  to  show  the  results  of  stimu- 
lating different  parts.  The  Sulcus  Centralis  is  the  fissure  of  Rolando.  (Front 
GRCNBAUM  AND  SHERRINGTON.) 

figure  more  largely  in  our  consciousness  than  the  physical 
changes  which  are  their  basis,  we  are  accustomed  to  assume 
that  the  movements  produced  are  the  result  of  volition,  and 
to  speak  of  them  as  voluntary  movements,  and  of  the  brain 
mechanism  producing  them  as  voluntary  centres.  There  is 
no  harm  in  doing  so  if  we  remember  that  these  centres  can  and 
do  act  without  the  involvement  of  consciousness,  and,  there- 
fore, without  volition ;  but  that  their  action  generally  implies 
the  previous  action  of  parts  of  the  receiving  and  associating 
mechanism  of  the  cerebrum. 

But  certain  lesions  may  directly  stimulate  these  centres, 


THE   NERVOUS  SYSTEM  185 

causing  them  to  act  without  the  previous  action  of  the  other 
cerebral  mechanisms.  This  is  seen  in  Jacksonian  epilepsy, 
where,  as  the  result  of  it  may  be  a  spicule  of  bone  or  a 
thickened  bit  of  membrane,  one  part  of  the  cortex  is  from 
time  to  time  excited,  and  by  its  action  produces  movement 
of  certain  groups  of  muscles. 

Experimental  observations  have  fully  confirmed  and 
extended  the  conclusions  arrived  at  from  such  pathological 
evidence. 

If  parts  of  these  convolutions  be  excised  in  the  monkey, 
the  animal  loses  the  power  of  voluntary  movement  of  certain 
groups  of  muscles,  while  if  they  are  stimulated  by  electricity 
these  groups  of  muscles  respond. 

These  Rolandic  convolutions,  in  front  of  the  fissure,  may  be 
considered  as  a  map  of  the  various  muscular  combinations 
throughout  the  body,  the  map  being  mounted  so  that  the 
lower  part  represents  the  face,  the  middle  part  the  arm,  and 
the  upper  part  the  leg.  Each  large  division  is  filled  in  so 
that  all  the  various  combinations  of  muscular  movement  are 
represented.  (Figs.  98  and  99.)  It  must  be  remembered 
that  these  centres  do  not  send  nerves  to  single  muscles,  but 
act  upon  groups  to  produce  definite  movements,  through  the 
lower  spinal  centres,  and  their  action  may  involve  not  merely 
stimulation  of  some  muscles  but  inhibition  of  others.  (See 
p.  60.) 

In  these  areas  the  lesion  must  be  extensive  to  cause  com- 
plete paralysis  of  any  group  of  muscles.  A  limited  lesion 
may  simply  cause  a  loss  of  the  finer  movements.  Thus,  a 
monkey  with  part  of  the  middle  portion  of  the  Rolandic  areas 
removed  may  be  able  to  move  its  arm  and  hand  about,  but 
may  be  quite  unable  to  pick  up  objects  from  the  floor  of 
its  cage. 

As  in  the  case  of  the  receiving,  so  in  that  of  the  discharg- 
ing mechanism,  it  is  the  most  recently  acquired  functions 
which  are  most  easily  lost.  This  is  well  illustrated  by  the 
results  of  lesions  of  the  left  inferior  frontal  convolution — 
the  area  which  presides  over  movements  of  the  lips  and 
tongue.  This  is  the  centre  which  has  to  be  specially  educated 
in  the  use  of  spoken  language ;  and,  when  this  region  is  only 
slightly  injured,  the  power  of  using  language  may  be  lost 


1 86  HUMAN   PHYSIOLOGY 

without  there  being  any  impairment  in  the  power  of  moving 
the  muscles.  This  was  long  ago  observed  by  Broca,  and  hence 
this  part  of  the  brain  is  often  called  Broca's  convolution. 
Similarly,  in  the  hand  area,  while  crude  lesions  are  required 
to  cause  loss  of  power  of  moving  the  hand,  very  slight  inter- 
ferences with  nutrition  may  cause  loss  of  power  of  expressing 
language  in  writing. 


PART    II 
NUTRITION   OF  THE   TISSUES 

SECTION    VI 

FLUIDS  BATHING  THE  TISSUES 

BLOOD  AND   LYMPH 

THE  blood  carries  the  necessary  nourishment  to  the  tissues, 
and  receives  their  waste  products.  But  it  is  enclosed  in  a 
closed  system  of  vessels,  and  does  not  come  into  direct  rela- 
tionship with  the  cells.  Outside  the  blood  vessels,  and  bath- 
ing the  cells,  is  the  lymph  which  plays  the  part  of  middleman 
between  the  blood  and  the  tissues,  receiving  nourishment 
from  the  former  for  the  latter,  and  passing  the  waste  from 
the  latter  into  the  former. 

A.  BLOOD. 

The  various  physical,  chemical,  and  histological  characters 
of  blood  must  be  practically  investigated. 

I.  General  Characters. 

Colour. — Blood  changes  its  colour  from  purple  to  cinnabar 
red  on  shaking  with  air,  showing  that  the  pigment  of  the 
blood  may  exist  in  two  conditions.  Elements  of  Blood. — 
Microscopic  examination  shows  that  blood  is  composed  of  a 
clear  fluid  (Liquor  Sanguinis  or  Plasma)  in  which  float 
myriads  of  small  disc-like  yellowish-red  cells  (Erythrocytes), 
and  a  smaller  number  of  greyish  cells  (Leucocytes),  and 
certain  more  minute  grey  particles  (Blood  Platelets).  The 
Opacity  of  Blood  is  due  to  the  erythrocytes.  When  the 
pigment  is  dissolved  out  of  them  by  water,  and  they  are 


1 88  HUMAN   PHYSIOLOGY 

rendered  transparent,  the  blood  as  a  whole  becomes  trans- 
parent and  "  laked."  The  Specific  Gravity  is  about  1055.  It 
may  be  estimated  by  finding  the  specific  gravity  of  a  sodium 
sulphate  solution  in  which  a  drop  of  blood  neither  sinks  nor 
floats. 

Taste  and  Smell  are  characteristic,  and  must  be  experi- 
enced. Reaction. — Blood  is  alkaline,  arid  the  degree  of 
alkalinity  is  very  constant  in  health.  It  is  equivalent  to 
about  0*3  per  cent,  of  Na2C03.  It  is  increased  during 
digestion,  and  diminished  after  muscular  exercise. 

Clotting  or  Coagulation. — In  the  course  of  about  three 
minutes  the  blood  when  shed  becomes  a  solid  jelly.  The 
process  starts  from  the  sides  of  the  vessel,  and  spreads 
throughout  the  blood  until,  when  clotting  is  complete,  the 
vessel  may  be  inverted  without  the  blood  falling  out.  In  a 
short  time  drops  of  clear  fluid  appear  upon  the  surface  of  the 
clot^and  in  a  few  hours  these  have  accumulated  to  a  con- 
siderable extent,  while  the  clot  has  contracted  and  drawn 
away  from  the  sides  of  the  vessel,  until  it  finally  floats  in  the 
clear  fluid — the  Serum.  Clotting  is  due  to  changes  in  the 
plasma,  since  this  fluid  will  coagulate  in  the  absence  of 
corpuscles. 

The  change  may  be  represented  thus  :— 

Blood 


i  •  i 

Plasma          Corpuscles 


Serum          Clot 

The  change  consists  in  the  formation  of  a  series  of  fine 
elastic  threads  of  fibrin  throughout  the  plasma,  and  if  red 
corpuscles  are  present  they  are  entangled  in  the  meshes  of 
the  network  and  give  the  clot  its  red  colour. 

These  threads  may  be  readily  collected  in  mass  upon  a 
stick  with  which  the  blood  is  whipped  as  it  is  shed.  The 
red  fluid  blood  which  is  left,  consisting  of  blood  cells  and 
serum,  is  said  to  be  defibrinated. 


THE   BLOOD  189 

Fibrin  is  a  proteid  substance.  It  is  slowly  dissolved  in 
solutions  of  neutral  salts.  It  is  coagulated  by  heat,  and  is 
precipitated  when  an  excess  of  a  neutral  salt  is  added.  It 
is  therefore  a  globulin. 

The  plasma  before  clotting  and  the  serum  squeezed  out 
from  the  clot  agree  in  containing  an  albumin  (serum  albumin) 
and  a  globulin,  or  series  of  globulins  which  may  be  classed 
together  as  serum  globulin,  in  the  same  proportion  in  each. 
But  the  plasma  contains  a  small  quantity — about  0*2  per 
cent. — of  another  globulin  (fibrinogen)  which  coagulates  at  a 
low  temperature,  and  which  is  absent  from  serum.  It  is  this 
which  undergoes  the  change  from  the  soluble  form  to  the 
insoluble  form  in  coagulation.  When  separated  from  the 
other  proteids  it  can  still  be  made  to  clot. 

The  substance  which  usually  causes  clotting  appears  to  be 
an  enzyme,  which  is  formed  by  the  union  of  a  nucleo-proteid 
or  a  derivative  of  a  nucleo-proteid  with  a  lime  salt.  The 
enzyme  may  be  called  thrombin,and  its  precursor  prothrom- 
bin.  Oxalates,  when  added  to  blood,  precipitate  the  soluble 
lime  salts,  and  prevent  the  formation  of  thrombin,  and  thus 
prevent  coagulation. 

There  is  some  evidence  that  the  prothrombin  exists  in 
solution  in  the  plasma,  but  it  certainly  is  derived  from  the 
breaking  down  of  the  cells  of  the  blood.  It  is  also  formed 
from  the  breaking  down  of  the  cells  of  such  tissues,  as 
lymphatic  glands  and  thymus. 

Many  circumstances  influence  the  rapidity  of  clotting. 
Temperature  has  a  marked  effect ;  a  low  temperature  re- 
tarding it,  a  slight  rise  of  temperature  above  the  normal  of 
the  particular  animal  accelerating  it.  If  a  trace  of  a  neutral 
salt  be  added  to  blood,  coagulation  is  accelerated ;  but  if  blood 
be  mixed  with  strong  solutions  of  salt,  coagulation  is  pre- 
vented. Salts  of  lime  have  a  marked  and  important  action, 
and  if  they  are  precipitated  by  the  addition  of  oxalate  of 
soda,  blood  will  not  clot,  apparently  because  thrombin  cannot 
be  formed. 

The  injection  into  the  blood  vessels  of  a  living  animal  of 
commercial  peptones,  which  chiefly  consist  of  proteoses,  or 
of  an  extract  of  the  head  of  the  medicinal  leech,  retards 
coagulation  after  the  blood  is  shed.  They  appear  to  cause 


1 90  HUMAN   PHYSIOLOGY 

the  development  in  the  liver  of  some  body  which  retards 
coagulation,  and  if  the  liver  be  excluded  from  the  circulation 
they  are  incapable  of  acting. 

Why  is  it  that  blood  does  not  coagulate  in  the  vessels  and 
does  coagulate  when  shed  ?  Such  a  general  statement  is 
not  absolutely  correct,  for  blood  may  be  made  to  coagulate 
in  the  vessels  of  a  living  animal  in  various  ways.  If  inflam- 
mation is  induced  in  the  course  of  a  vessel,  coagulation  at 
once  occurs.  If  the  inner  coat  of  a  vessel  be  torn,  as  by  a 
ligature,  or  if  any  roughness  occurs  on  the  inner  wall  of  a 
vessel,  coagulation  is  apt  to  be  set  up.  Again,  various  sub- 
stances injected  into  the  blood  stream  may  cause  the  blood 
to  coagulate,  and  thus  rapidly  kill  the  animal.  Among  such 
substances  are  extracts  of  various  organs — thymus,  testis, 
and  lymph  glands — which  yield  prothrombin,  although 
curiously  enough  the  injection  of  pure  thrombin  does  not 
usually  act  in  this  way.  Nor  does  blood  necessarily  coagulate 
when  shed.  If  it  is  received  into  castor  oil,  or  into  a  vessel 
anointed  with  vaseline  and  filled  with  paraffin  oil,  it  will 
remain  fluid  for  a  considerable  time. 

Apparently  some  roughness  in  the  wall  of  the  blood  vessel 
or  of  the  vessel  in  which  the  blood  is  received  is  required  to 
start  the  process,  acting  as  a  focus  from  which  it  can  spread 
outwards. 

The  advantages  of  coagulation  of  blood  are  manifest. 
By  means  of  it  wounds  in  blood  vessels  are  sealed  and 
haemorrhage  stopped. 

Although  an  important  and  very  prominent  change  in 
the  blood,  clotting  is  really  produced  by  change  in  one 
constituent  of  the  plasma,  which  is  present  in  very  small 
quantities. 

II.  Plasma  and  Serum. 

These  may  be  considered  together,  since  serum  is  merely 
plasma  minus  fibrinogen.  As  serum  is  so  much  easier  to 
procure,  it  is  generally  employed  for  analysis. 

Both  are  straw-coloured  fluids,  the  colour  being  due  to  a 
yellow  lipochrome.  Sometimes  they  are  clear  and  trans- 
parent, but  after  a  fatty  diet  they  become  milky.  They  are 
alkaline  in  reaction,  and  have  a  specific  gravity  of  about 


THE  BLOOD  191 

1025.  They  contain  about  90  per  cent,  of  water  and  10  per 
cent,  of  solids.  The  chief  solids  are  the  proteids — serum 
albumin  and  serum  globulin  (with,  in  the  plasma,  the  addition 
of  fibrinogen).  The  proportion  of  the  two  former  proteids 
to  one  another  varies  considerably  in  different  animals,  but 
in  the  same  animal  at  different  times  the  variations  are 
small.  The  globulin  probably  consists  of  at  least  two 
bodies — englobulin  precipitated  by  weak  acid,  and  pseudo- 
globulin  not  so  precipitated.  The  amount  of  albumin  is 
generally  greater  when  the  body  is  well  nourished.  In  man, 
they  together  form  about  7  per  cent,  of  the  serum. 

The  other  organic  constituents  of  the  serum  are  in  much 
smaller  amounts  and  may  be  divided  into — 

1.  Substances  to  be  used  by  the  tissues. 

Glucose  is  the  most  important  of  these.  It  occurs  only  in 
small  amounts — about  1  to  2  per  mille.  Part  of  it  is  free, 
but  part  is  probably  combined  in  organic  combinations  such 
as  jecorin.  It  is  probably  in  larger  amount  in  blood  going 
to  muscles  than  in  blood  coming  from  muscles,  and  this 
difference  seems  to  be  specially  well  marked  when  the 
muscles  are  active. 

Fats  occur  in  very  varying  amounts,  depending  upon  the 
amount  taken  in  the  food. 

2.  Substances  given  off  by  the  tissues. 

The  chief  of  these  is  urea,  which  occurs  constantly  in 
very  small  amounts  in  the  serum — about  -05  per  cent. 
We  shall  afterwards  see  that  it  is  derived  from  the  liver, 
and  that  it  is  excreted  in  the  urine  by  the  kidneys. 

Creatin  (p.  43),  uric  acid  (p.  397),  and  some  allied  bodies 
appear  to  be  normally  present  in  traces,  and  their  amount 
may  be  increased  in  diseased  conditions. 

Of  the  inorganic  constituents  of  the  serum  the  most 
abundant  is  chloride  of  sodium,  but  in  addition  sodium 
carbonate,  and  alkaline  sodium  phosphate,  are  also  present. 
Calcium,  potassium  and  magnesium  occur  in  very  small 
amounts. 


192 


HUMAN   PHYSIOLOGY 


III.  Cells  of  Blood. 

1.  Leucocytes  —  White  Cells.  —  These  are  much  less 
numerous  than  the  red  cells,  and  their  number  varies 
enormously  in  normal  conditions.  On  an  average  there 
are  about  7500  per  cubic  millimetre. 

They  are  soft,  extensile,  elastic,  and  sticky,  and  each 
contains  a  nucleus  and  a  well-developed  double  centrosome. 
In  size  they  vary  considerably,  some  being  much  larger  than 
the  red  cells,  some  slightly  smaller.  The  character  of  the 


FIG.  100.— Cells  of  the  Blood,     (a)  Erythrocytes ;   (b)  Large,  and  (c)  Small  Lym- 
phocyte ;  (d)  Polymorpho-uuclear  Leucocyte ;  (e)  Eosinophil  Leucocyte. 

nucleus  varies  greatly,  and  from  this  and  variations  in  the 
protoplasm,  they  may  be  divided  into  four  classes. 

1st.  Lymphocytes. — Cells  with  a  clear  protoplasm  and  a 
more  or  less  circular  nucleus.  Some  are  very  small,  while 
others  are  larger.  They  constitute  about  20  to  25  per  cent, 
of  the  leucocytes  (Fig.  100,  b  and  c). 

2nd.  Polymorpho-nuclear  leucocytes,  with  a  much  distorted 
and  lobated  irregular  nucleus  and  a  finely  granular  proto- 
plasm, whose  granules  stain  with  acid  and  neutral  stains. 
These  constitute  about  70  to  75  per  cent,  of  the  leucocytes. 

3rd.  Eosinophil  or  oxyphil  leucocytes,  with  a  lobated 
nucleus  like  the  last,  but  with  large  granules  in  the  proto- 
plasm which  stain  deeply  with  acid  stains.  From  1  to  4 
per  cent,  of  the  leucocytes  are  of  this  variety. 


THE   BLOOD  193 

4th.  Basophil  leucocytes,  practically  absent  from  normal 
blood,  with  a  lobated  nucleus  and  granules  in  the  protoplasm, 
staining  with  basic  stains. 

Myelocytes  are  large , leucocytes  with  a  large  circular  or 
oval  nucleus  and  a  finely  granular  protoplasm.  They  are 
not  normal  constituents  of  the  blood,  but  appear  when  the 
activity  of  the  bone  marrow  is  increased  in  certain  patho- 
logical conditions. 

These  various  forms  have  certain  properties — (a)  Amoeboid 
movement.  They  can,  under  suitable  conditions,  undergo 
changes  in  shape,  as  may  be  readily  seen  in  the  blood  of  the 
frog  or  other  cold-blooded  animal.  The  motion  may  consist 
simply  of  the  pushing  out  and  withdrawal  of  one  or  more 
processes  (pseudopodia),  or,  after  a  process  is  extended,  the 
whole  corpuscle  may  follow  it  and  thus  change  its  place,  or 
the  corpuscle  may  simply  retract  itself  into  a  spherical  mass. 
As  a  result  of  these  movements  the  corpuscles,  in  certain  con- 
ditions, creep  out  of  the  blood  vessels  and  wander  into  the 
tissues  (Diapedesis). 

(6)  Phagocyte  action. — The  finely  granular  leucocytes  and 
the  lymphocytes  have  further  the  power  of  taking  foreign 
matter  into  their  interior,  and  of  thus  digesting  it.  By  this 
devouring  action,  useless  and  effete  tissues  are  removed  and 
dead  micro-organisms  in  the  body  are  taken  up  and  got  rid  of. 
In  disposing  of  these  micro-organisms  the  large  granular  cor- 
puscles seem  also  to  play  an  important  part.  This  scavenger 
action  of  the  leucocytes  is  of  vast  importance  in  pathology. 

Chemistry  of  Leucocytes. — The  nucleus  is  chiefly  made  up 
of  nuclein,  and  in  the  protoplasm  a  nucleo-albumin,  along 
with  two  globulins  and  a  small  amount  of  an  albumin,  are 
found.  Along  with  these  proteid  substances  glycogen  and  a 
small  amount  of  fat  are  present,  while  the  chief  inorganic 
constituents  are  potassium  salts. 

2.  Blood  Platelets. — These  are  small  circular  or  oval  dis- 
coid bodies,  about  one-third  the  diameter  of  a  red  blood  cor- 
puscle. Some  observers  have  stated  that  they  contain  a 
central  nucleus.  They  are  very  sticky,  and  mass  together 
when  blood  is  shed  and  adhere  to  a  thread  passed  through 
blood  or  to  any  rough  point  in  the  lining  of  the  heart  or 

13 


194  HUMAN   PHYSIOLOGY 

vessels.  They  there  form  clumps,  and  from  these  clumps 
fibrin  threads  are  seen  to  shoot  out.  They  thus  appear  to 
play  an  active  part  in  clotting,  possibly  by  yielding  thrombin. 
They  are  present  in  the  blood  of  mammals  only.  Their  source 
is  not  definitely  known,  but  it  has  been  suggested  that  they 
are  the  extruded  nuclei  of  developing  erythrocytes. 

3.  Erythrocytes — Red  Cells. — Shape.  All  mammals  ex- 
cept the  camels  have  circular,  biconcave,  discoid  erythrocytes, 
which,  when  the  blood  is  shed,  tend  to  run  together  like  piles 
of  coins.  The  camels  have  elliptical  biconvex  corpuscles.  A 
nucleus  is  not  present  in  the  fully  developed  mammalian 
erythrocyte.  In  birds,  reptiles,  amphibia  and  fishes,  the 
corpuscles  are  elliptical  biconvex  bodies,  with  a  well-marked 
central  nucleus.  The  size  of  the  human  erythrocytes  is  fairly 
constant — on  an  average  7 '6  micro-millimetres  in  diameter. 
The  number  of  red  cells  in  health  is  about  5,000,000  in 
men  and  4,500,000  in  women,  per  cubic  millimetre ;  but  in 
disease  it  is  often  decreased. 

The  number  of  corpuscles  per  cubic  millimetre  is  estimated 
by  the  Haeinocytometer.  This  consists  of  (1)  a  pipette  by 
which  the  blood  may  be  diluted  to  a  definite  extent  with 
normal  salt  solution,  and  (2)  a  cell  ruled  in  squares  each  con- 
taining above  it  a  definite  small  volume  of  blood  so  that  the 
number  of  corpuscles  in  that  volume  may  be  counted  under 
the  microscope.  (Experiment.) 

The  pale  yellow  colour  of  the  individual  corpuscles  is  due 
to  the  pigment  they  contain  being  spread  out  in  so  thin  a 
layer.  This  pigment  may  be  dissolved  out  by  various  agents 
— e.g.  salts  of  the  bile  acids,  distilled  water,  dilute  acids,  or 
alkalies,  &c.,  leaving  a  colourless  shadow-like  corpuscle,  which 
seems  to  be  composed  of  a  sponge-like  stroma  in  the  inter- 
stices of  which  the  pigment  is  held. 

Chemistry. — The  stroma  of  the  erythrocytes  is  made  up  of 
a  globulin-like  substance,  in  connection  with  which  lecithin 
and  cholesterin  occur  in  considerable  quantities.  Potassium 
is  the  base  most  abundantly  present.  This  stroma  seems  to 
form  a  capsule  round  the  cell,  and  through  this  capsule  water 
can  pass  under  the  influence  of  osmotic  pressure.  It  is  well 
known  that,  if  two  solutions  of  different  molecular  concen- 


THE   BLOOD  195 

tration  are  separated  by  an  animal  membrane,  water  will 
pass  from  that  of  lower  concentration  into  that  of  higher 
concentration.  Hence  if  blood  corpuscles  are  placed  in  a 
series  of  saline  solutions,  it  will  be  found  that,  if  the  solution 
is  of  lower  molecular  concentration  than  the  corpuscle,  fluid 
will  pass  in,  swell  the  corpuscle,  and  perhaps  burst  it  and  set 
free  the  pigment,  while,  if  the  fluid  is  of  higher  concentra- 
tion, the  fluid  will  pass  from  the  corpuscle  which  will  shrivel. 
Hence  the  change  in  the  corpuscle  is  used  as  a  means  of 
determining  the  osmotic  pressure  and  molecular  concentra- 
tion of  various  solutions. 

The  pigment  is  Haemoglobin.  It  constitutes  no  less  than 
90  per  cent,  of  the  solids  of  the  corpuscles.  In  many  animals 
when  dissolved  from  the  corpuscles  this  substance  tends  to 
crystallise  very  readily.  The  crystals  prepared  from  the 
human  blood  are  rhombic  plates.  When  exposed  to  air  they 
are  of  a  bright  red  colour,  but  if  placed  in  the  receiver  of  an 
air-pump  at  the  ordinary  temperature  they  become  of  a  pur- 
plish tint.  The  same  thing  occurs  if  the  haemoglobin  is  in 
solution,  or  if  it  is  still  in  the  corpuscles.  The  addition  of 
any  reducing  agent  such  as  sulphide  of  ammonia  or  a  ferrous 
salt  also  causes  a  similar  change.  This  is  due  to  the  fact  that 
haemoglobin  has  an  affinity  for  oxygen,  which  it  takes  up  from 
the  air,  forming  a  definite  compound  of  a  bright  red  colour  in 
which  one  molecule  of  haemoglobin  links  with  a  molecule  of 
oxygen,  Hb02,  and  is  known  as  oxyhaemoglobin. 

Haemoglobin  is  closely  allied  to  the  proteids,  but  differs 
from  them  in  containing  0*42  per  cent,  of  iron. 

When  light  from  the  sun  is  allowed  to  pass  through  these 
solutions  certain  parts  of  the  spectrum  are  absorbed,  and 
when  the  spectrum  is  examined  dark  bands — the  absorption 
spectra — are  seen.  In  a  weak  solution  of  oxyhaemoglobin-  a 
dark  band  is  seen  in  the  green  and  another  in  the  yellow  part 
of  the  spectrum  between  Frauenhofer's  lines  D  and  E,  while 
the  violet  end  of  the  spectrum  is  absorbed  (Fig.  101).  These 
bands  may  be  broadened  or  narrowed  by  strengthening  or 
weakening  the  solution.  When  the  oxygen  is  taken  away  and 
the  purple  reduced  haemoglobin  is  formed,  a  single  broad  band 
between  D  and  E  takes  the  place  of  the  two  bands  (Fig.  101), 

The  property  of  taking  oxygen  from  the  air  and  of  again 


196  HUMAN  PHYSIOLOGY 

giving  it  up  at  a  moderate  temperature  and  under  a  low 
pressure  of  oxygen  is  the  great  function  of  the  blood  pigment 
in  the  body.  The  haemoglobin  plays  the  part  of  a  middle- 
man between  the  air  and  the  tissues,  taking  oxygen  from  the 
one  and  handing  it  on  to  the  others,  just  as  it  may  be  made 
to  act  when  in  solution  in  a  test  tube  containing  sulphide  of 
ammonia  or  a  ferrous  salt.  (Chemical  Physiology,  p.  14.) 

Haemoglobin  constitutes  about  13  or  14  per  cent,  of  the 
blood,  but  in  various  diseases  its  amount  is  decreased.  The 
best  method  of  estimating  its  amount  is  by  Haldane's  Hsemo- 
globino meter.  This  consists  of  two  tubes  of  uniform  calibre, 
one  tilled  with  a  1  per  cent,  solution  of  normal  blood 
saturated  with  CO,  and  another  in  which  20  cmm.  of  blood 
to  be  examined,  measured  in  a  pipette,  is  placed  in  water, 
mixed  with  coal  gas  to  saturate  with  CO,  and  then  diluted 
till  it  has  the  same  tint  as  the  standard  tube.  The  per- 
centage of  haemoglobin  in  terms  of  the  normal  is  indicated  by 
the  mark  on  the  tube  at  which  the  fluid  stands.  (Chemical 
Physiology,  p.  15.) 

Methaemoglobin. — Haemoglobin  forms  another  compound 
with  oxygen — methaemoglobin.  The  amount  of  oxygen  is 
the  same,  but  methaemoglobin  must  be  acted  on  by  the 
strongest  reducing  agents  before  it  will  part  with  its  oxygen. 
When  therefore  this  pigment  is  formed  in  the  body,  the 
tissues  die  for  want  of  oxygen.  It  may  be  produced  by  the 
action  of  various  substances  on  oxyhsemoglobin.  Among 
these  are  ferricyanides,  nitrites,  and  permanganates.  It 
crystallises  in  the  same  form  as  oxyhaemoglobin,  but  has  a 
chocolate  brown  colour.  Its  spectrum  is  also  different  from 
haemoglobin  or  oxyhaemoglobin,  showing  a  narrow  sharp  band 
in  the  red  part  of  the  spectrum,  with  two  or  more  bands  in 
other  parts  according  to  the  reaction  of  the  solution  in  which 
it  is  dissolved  (Fig.  101).  It  is  of  importance  since  it  occurs  in 
the  urine  in  some  pathological  conditions.  In  all  probability 

the  molecule  of  oxyhaemoglobin  has  the  formula — Hb/ 

X) 

while  in  methaemoglobin  the  atoms  are  arranged 


THE  BLOOD 


197 


Haemoglobin  also  combines  with  certain  other  gases. 
Among  these  is  Carbon  monoxide.  For  this  gas  haemoglobin 
has  a  greater  affinity  than  for  oxygen,  so  that  when  carbon 
monoxide  haemoglobin  is  once  formed  in  the  body,  the 
blood  has  little  power  of  taking  up  oxygen,  and  the  animal 
dies.  This  gas  is  evolved  freely  in  the  fumes  from  burn- 
ing charcoal,  and  the  production  of  the  compound  in  the 
blood  is  the  cause  of  death  from  inhaling  such  fumes  in 


RED 


YELLOW.    GREEN. 


BLUE. 


Carbon-monoxide  ^    f 
Hemoglobin     .  >    f 


I 

j 

, 

-  • 
•     '      i 

Haemoglobin         .     | 

t 

Methfemoglobin  .  1  : 
Acid  Hjematin     .  /  j 

1 

,?.±j 

. 

Reduced  Alkaline  \ 
Hajmatin   .        .  /  i 

i    IL 

FIG.  101. — Spectra  of  the  more  important  Blood  Pigments  and  their  more  important 
derivatives.  (The  Spectrum  of  Acid  Haematin  is  not  identical  with  that  of 
Methsemoglobin). 

closed  spaces.  It  is  also  found  in  the  air  of  coal  mines 
after  explosions.  The  carbon  monoxide  haemoglobin  forms 
crystals  like  oxy haemoglobin,  and  has  a  bright  pinkish 
red  colour,  without  the  yellow  tinge  of  oxy haemoglobin. 
Since  after  death  it  does  not  give  up  its  carbon  monoxide 
and  become  changed  to  purple  haemoglobin,  the  bodies  of 
those  poisoned  with  the  gas  maintain  the  florid  colour  of 
life.  Its  spectrum  is  very  like  that  of  oxyhaemoglobin,  the 
bands  being  slightly  more  to  the  blue  end  of  the  spectrum 
(Fig.  101). 


198  HUMAN  PHYSIOLOGY 

It  may  be  at  once  distinguished  by  the  fact  that  when 
gently  warmed  with  sulphide  of  ammonia  it  does  not  yield 
reduced  haemoglobin.  (Chemical  Physiology,  p.  14.) 

Decomposition  of  Haemoglobin. — Haemoglobin  is  a  some- 
what unstable  body,  and,  in  the  presence  of  acids  and  alkalies, 
splits  up  into  about  96  per  cent,  of  a  colourless  proteid  sub- 
stance belonging  to  the  globulin  group,  and  about  4  per  cent, 
of  a  substance  of  a  brownish  colour  called  haematin.  The 
spectrum  and  properties  of  this  substance  are  different  in 
acid  and  alkaline  media.  In  acid  media  it  has  a  spectrum 
closely  resembling  methaemoglobin,  but  it  can  at  once  be 
distinguished  by  the  fact  that  it  is  not  changed  by  reducing 
agents.  In  medicine  it  is  sometimes  important  to  distinguish 
between  these  pigments.  Haematin  in  alkaline  solution 
can  take  up  and  give  off  oxygen  in  the  same  way  as 
haemoglobin  does.  Reduced  alkaline  haematin  or  hsemo- 
chromogen  has  a  very  definite  spectrum  (Fig.  101),  and  its 
preparation  affords  a  ready  means  of  detecting  old  blood 
stains.  Haematin  contains  the  iron  of  the  haemoglobin,  and 
it  is  this  pigmented  iron-containing  part  of  the  molecule 
which  has  the  affinity  for  oxygen.  Apparently  it  is  the  pre- 
sence of  iron  which  gives  it  this  property,  because,  if  the  iron 
be  removed  by  means  of  sulphuric  acid,  a  purple-coloured 
substance,  iron-free  hcematin,  haematoporphyrin,  is  formed, 
which  has  no  affinity  for  oxygen.  This  pigment  occurs  in 
the  urine  in  some  pathological  conditions.  (Chemical  Physi- 
ology, p.  15.) 

In  the  liver  haemoglobin  is  broken  down  to  form  bilirubin 
and  the  other  bile  pigments.  These  are  iron-free,  and,  like 
iron-free  haematin,  do  not  take  up  and  give  off'  oxygen.  But 
not  only  is  this  iron-free  pigment  formed  from  haemoglobin 
in  the  liver,  but  the  cells  of  any  part  of  the  body  have  the 
faculty  of  changing  haemoglobin  in  blood  extravasations  into 
a  pigment  known  as  haematoidin,  which  is  really  the  same  as 
bilirubin. 

Haemin — the  hydrochloride  of  haematin — is  formed  when 
blood  is  heated  with  chloride  of  sodium  and  glacial  acetic 
acid.  It  crystallises  in  small  steel  black  rhombic  crystals, 
and  its  formation  is  sometimes  used  as  a  test  of  blood  stains. 
(Chemical  Physiology,  p.  16.) 


THE   BLOOD 


199 


The  following  table  shows  the  relationship  of  these  pig- 
ments to  one  another : — 

RELATIONSHIP  OF  HB  AND  ITS  DERIVATIVES. 
Methsemoglobin Hb02 HbCO 

Hb 


Haematin 


Globin 


Acid  Hsematin 


Alkaline  Hsematin 


Oxidised      Reduced 

(Hsemochromogen) 


Contain  Iron. 


Iron-free  Haematin 
(Hsematoporphyrin) 


Haamatoidin 
Bilirubin 


j-  Iron-free. 


IY.  Gases  of  the  Blood. 

The  muscles  and  other  active  tissues  are  constantly  con- 
suming oxygen  and  constantly  giving  off  carbon  dioxide. 
The  oxygen  must  be  brought  to  the  tissues  by  the  blood, 
and  the  carbon  dioxide  carried  away  by  the  same  medium. 

Various  methods  of  carrying  out  the  examination  of  the 
gases  of  the  blood  have  been  devised,  and  many  different  gas 
pumps  have  been  invented  in  which  the  gases  may  be  col- 
lected in  the  Torricellian  vacuum  over  mercury.  Haldane  and 
Barcroft  have  devised  a  convenient  method,  which  depends 
upon  the  fact  that  the  oxygen  can  be  driven  off  from  blood 
treated  with  dilute  ammonia  by  the  addition  of  ferricyanide 
of  potassium,  and  that  the  carbon  dioxide  is  liberated  by 
adding  an  acid,  and  the  amount  of  gas  estimated  by  measur- 
ing the  increased  pressure  in  the  tube  in  which  the  gas  has 
been  given  off. 

About  60  c.c.  of  gas  measured  at  0°  C.  and  760  mm.  pres- 
sure can  be  extracted  from  100  c.c.  of  blood.  The  proportion 
of  the  gases  varies  in  arterial  and  in  venous  blood. 


200  HUMAN  PHYSIOLOGY 


AMOUNT  OF  GASES  PER  HUNDRED  VOLUMES  OF  BLOOD. 

Arterial  Blood.      Venous  Blood. 

Oxygen  20  12 

Carbon  dioxide   .         40  46 

There  are  two  ways  in  which  gases  may  be  held  in  such  a 
fluid  as  the  blood— 

1st.  In  simple  solution. 

2nd.  In  chemical  combination. 

Oxygen. — At  the  temperature  of  the  body  the  blood  can 
hold  in  solution  less  than  1  per  cent,  of  oxygen.  Now  the 
amount  of  oxygen  actually  present  is  about  20  per  cent.  So 
that  by  far  the  greater  quantity  of  the  gas  is  not  in  solution. 
We  have  already  seen  that  it  is  in  loose  chemical  union  with 
hsemoglobin. 

Carbon  dioxide. — In  the  animal  body  the  blood  can  dis- 
solve about  2J  per  cent,  of  carbon  dioxide.  But  it  may  con- 
tain as  much  as  46  per  cent.  Hence  the  greater  part  of  the 
gas  must  be  in  chemical  combination.  Since  the  proportion 
of  carbon  dioxide  is  greater  in  the  plasma  than  in  entire 
blood,  this  gas  must  be  held  in  the  plasma.  Analysis  of  the 
ash  of  the  plasma  shows  that  the  sodium  is  more  than 
sufficient  to  combine  with  the  chlorine  and  phosphoric  acid, 
and  is  thus  available  to  take  up  carbon  dioxide.  Carbonate 
of  soda  and  basic  phosphate  of  soda  are  therefore  present 
together  in  the  plasma,  and  if  a  stream  of  carbon  dioxide  be 
passed  through  a  solution  of  basic  phosphate  of  soda,  it 
appropriates  a  certain  amount  of  soda  and  leaves  the  neutral 
phosphate.  On  the  other  hand,  if  the  amount  of  carbon 
dioxide  is  small,  the  phosphoric  acid  again  seizes  on  the  soda, 
and  turns  out  the  carbon  dioxide.  Thus  the  soda  is  the 
carrier  of  carbon  dioxide  in  the  blood  plasma.  The  blood, 
in  passing  through  the  tissues  which  are  loaded  with  this 
gas,  gains  carbon  dioxide,  and  some  of  the  basic  phosphate 
of  s*oda  is  changed  to  the  less  alkaline  neutral  phosphate. 
When  the  lungs  are  reached  the  blood  is  exposed  to  air  poor 
in  carbon  dioxide,  and  the  phosphoric  acid  is  able  to  turn 
out  the  carbon  dioxide.  In  fact,  the  carriage  of  carbon 


THE  BLOOD  201 

dioxide  and  its  excretion  are  the  result  of  a  struggle  between 
that  gas  and  phosphoric  acid  for  the  soda  of  the  plasma. 

Nitrogen. — The  amount  of  nitrogen  in  the  blood  is  not  in 
excess  of  what  can  be  held  in  solution ;  and  we  may  there- 
fore infer  that  it  is  simply  dissolved  in  the  blood  plasma. 

Y.  Source  of  the  Blood  Constituents. 

A.  Of  the  Plasma. — The  water  of  the  blood  is  derived 
almost  entirely  from  the  water  ingested. 

The  source  of  the  proteids  has  not  been  fully  investigated. 
Undoubtedly  they  are  partly  derived,  somewhat  indirectly  as 
we  shall  afterwards  see,  from  the  proteids  of  the  food.  Very 
probably,  too,  they  are  in  part  derived  from  the  tissues.  But 
the  significance  of  the  two  proteids,  albumin  and  globulin, 
and  their  variations  has  not  yet  been  elucidated. 

The  glucose  is  derived  from  the  carbohydrates  and  possibly 
from  the  proteids  of  the  food,  and  during  starvation  it  is 
constantly  produced  in  the  liver  and  poured  into  the  blood. 

The  fats  are  derived  from  the  fats  and  carbohydrates  and 
possibly  from  the  proteids  of  the  food. 

The  urea  and  other  waste  constituents  are  derived  from 
the  various  tissues. 

B.  Of  the  Cells — I.  Leucocytes. — These  are  formed  in  the 
lymph  tissue  and  in  the  red  marrow  of  bone. 

1.  Lymph  Tissue  (see  p.  30)  is  very  widely  distributed  in 
the  body,  occurring  either  in  patches  of  varying  shape  and  size, 
or  as  regular  organs,  the  lymphatic  glands  (Fig.  102).  These 
are  placed  on  the  course  of  a  lymphatic  vessel,  and  consist  of 
a  sponge-work  of  fibrous  tissue,  in  the  interstices  of  which  are 
set  the  patches  of  lymph  tissue  or  germ  centres,  each  sur- 
rounded by  a  more  open  network,  the  sinus,  through  which 
the  lymph  flows,  carrying  away  the  lymphocytes  from  the 
germ  centres.  Round  some  of  the  lymphatic  glands  of 
certain  animals  large  blood  spaces  or  sinuses  are  seen,  and 
these  glands  are  called  hsemolymph  glands.  They  are  inter- 
mediate between  lymphatic  glands  and  the  spleen.  While 
these  glands  produce  lymphocytes,  they  also  play  an  im- 
portant part  in  disposing  of  disintegrating  erythrocytes  and 
in  storing  iron.  * 


202  HUMAN   PHYSIOLOGY 

2.  Bone  Marrow. — The  structure  of  bone  marrow  is  con- 
sidered below,  but  it  may  be  stated  here  that  young  leuco- 
cytes or  leucoblasts,  in  the  condition  of  mitosis,  are  abundant, 
and  that  they  seem  to  pass  away  in  the  blood  stream.  They 
are  of  all  varieties.  In  certain  pathological  conditions  the 
formation  of  these  cells  is  increased  and  a  leucocytosis 
results  (Fig.  103). 

II.  Erythrocytes.— These  are  formed  after  birth  in  the 
red  marrow  of  bone.  Marrow  consists  of  a  fine  fibrous 
tissue  with  large  blood  capillaries  or  sinuses  running  in  it. 
In  the  fibrous  tissues  are  numerous  fat  cells,  and  generally 


FIG.  102. — Section  of  a  Lymph  Gland,    a.,  capsule ;  6.,  germ  centres  of  cortex  ; 
c.,  sinuses ;  d. ,  trabecula ;  e. ,  germ  centres  of  medulla. 


a  considerable  number  of  multi-nucleated  giant  cells.  In 
addition  to  these  are  the  young  leucocytes,  leucoblasts,  which 
used  to  be  called  the  proper  cells  of  the  bone  marrow,  and 
lastly  young  nucleated  red  cells,  the  erythroblasts.  After 
haemorrhage,  the  formation  of  these  becomes  unusually 
active,  and  may  implicate  parts  of  the  marrow  not  generally 
concerned  in  the  process,  and  hence  the  red  marrow  may 
spread  from  the  ends  of  the  long  bones,  where  it  is  usually 
situated,  towards  the  middle  of  the  shaft.  The  nuclei  of  the 
erythroblasts  atrophy  or  are  shed  and  the  cells  escape  into 
the  blood  stream.  The  red  marrow  has  the  power  of  re- 
taining the  iron  of  disintegrate'd  erythrocytes,  which  are 


THE  BLOOD  203 

often  found  enclosed  in  large  phagocytes.     The  iron  is  often 
very  abundant  after  a  marked  destruction  of  erythrocytes. 

YI.  Total  Amount  of  Blood  in  the  Body. 

This  was  formerly  determined  by  bleeding  an  animal, 
measuring  the  amount  of  blood  shed,  and  determining  the 
amount  of  haemoglobin  contained  in  it;  then  washing  out 
the  blood  vessels,  and  after  measuring  the  amount  of  fluid 


FlG.  103. — Section  of  Red  Marrow  of  Bone.  a. ,  lymphocyte  ;  b. ,  fat  cell ; 
c. ,  erythroblast ;  d.,  giant  cell;  e. ,  erythrocyte  ;  /.,'  erythroblast  in 
mitosis;  g. ,  neutrophil  myelocyte  ;  A.,  eosinophil  myelocyte  ;  k., 
eosinophil  leucocyte  ;  I. ,  polymorpho-nuclear  leucocyte. 

used,  determining  the  amount  of  haemoglobin  in  it  to  ascer- 
tain the  amount  of  blood  it  represents.  By  this  method 
the  amount  of  blood  was  found  to  be  about  TV  of  the  body 
weight. 

Haldane  and  Lorrain  Smith  have  devised  a  method  which 
can  be  applied  to  the  living  animal.  It  depends  upon  the 
fact  that  after  a  person  has  inhaled  carbon  monoxide  it  is 
possible  to  determine  to  what  proportion  the  gas  has  replaced 
oxygen  in  the  oxy haemoglobin.  If  then  an  individual  breathes 
a  given  volume  of  carbon  monoxide,  and  if  a  measured  speci- 
men of  blood  contains  a  definite  percentage  of  the  gas,  the 


204  HUMAN  PHYSIOLOGY 

rest   of  the  gas  must  be  equally  distributed   through   the 
blood,  and  thus  the  amount  of  blood  may  be  deduced. 

By  this  method  they  conclude  that  the  volume  of  the 
blood  is  about  ?\ih  of  the  weight  of  the  body. 


VII.  Distribution  of  the  Blood. 

Roughly  speaking,  the  blood  is  distributed  somewhat  as 
follows : — 

Heart,  lungs,  large  vessels  J 

Muscles  .......  J 

Liver      

Other  organs J 


YIII.  Fate  of  the  Blood  Constituents. 

The  water  of  the  blood,  constantly  renewed  from  outside, 
is  constantly  got  rid  of  by  the  kidneys,  skin,  lungs,  and 
bowels. 

About  the  fate  of  the  proteids  we  know  nothing.  They 
must  be  used  up  in  the  construction  of  the  tissues,  but 
experimental  evidence  of  this  is  wanting. 

The  glucose  and  fat  are  undoubtedly  used  up  in  the 
tissues. 

The  urea  and  waste  products  are  excreted  by  the  kidneys. 

The  fate  of  the  salts  is  not  fully  worked  out.  The  chlorides 
are  partly  excreted  by  the  kidneys  and  are  partly  split  up 
to  form  the  hydrochloric  acid  required  for  stomach  diges- 
tion. The  phosphates  and  sulphates  are  excreted  in  the 
urine,  but  whether  they  are  also  used  in  the  tissues  is  not 
known. 

The  leucocytes  break  down  in  the  body — but  when  and 
how  we  do  not  know.  We  shall  afterwards  find  that  they 
are  greatly  increased  in  number  after  a  meal  of  proteids,  and, 
since  the  increase  is  transitory,  lasting  only  for  a  few  hours, 
they  are  probably  rapidly  broken  down,  possibly  to  feed  the 
tissues.  It  would  thus  seem  that  a  leucocyte  lives  in  the 
blood  only  for  a  short  time. 

The  erythrocytes  also  break  down.  How  long  they  live 
is  not  known.  It  is  found,  after  injecting  blood,  that  the 


THE  BLOOD  205 

original  number  of  corpuscles  is  not  reached  for  about  a  fort- 
night, and  hence  it  has  been  concluded  that  the  corpuscles 
live  for  that  period.  The  experiment,  however,  is  far  from 
conclusive,  and  must  be  accepted  with  reservation. 

Organs  Connected  with  Haemolysis.  —  The  process  of 
breaking  down  old  erythrocytes  and  eliminating  their  pig- 
ment is  often  called  the  process  of  haemolysis.  Certain 
organs  appear  to  be  specially  connected  with  this,  but  the 
precise  part  played  by  each  of  them  is  not  very  clearly 
understood. 

That  the  liver  acts  in  this  way  is  indicated,  first,  by  the 


FIG.  104.— To  show  the  relationship  of  the  Spleen  to  Lymph  Glands  and 
Hsemolymph  Glands.  The  black  indicates  Lymphoid  Tissue ;  the 
coarsely  spotted  part  Lymph  Sinuses,  and  the  finely  dotted  part 
Blood  Sinuses.  (LEWIS.) 

fact  that  the  blood  passing  from  the  organ  during  digestion 
contains  fewer  erythrocytes  than  the  blood  going  to  it; 
second,  by  the  formation  of  bile  pigment,  a  derivative  of 
haemoglobin,  in  the  liver  cells;  third,  by  the  presence  of 
pigment  and  of  iron  in  simple  combinations  in  the  liver  cells 
under  certain  conditions.  It  is  possible  that  the  reabsorbed 
salts  of  the  bile  acids  in  the  portal  blood  may  dissolve  the 
pigment  out  of  the  old  erythrocytes,  and  that  the  liver  cells 
may  then  act  upon  the  liberated  pigment.  Under  ordinary 
conditions  the  liver  does  not  store  much  iron. 

The  spleen  is  generally  said  to  have  a  similar  action.    This 
organ  is  composed  of  a  fibrous  capsule  containing  non-striped 


206  HUMAN  PHYSIOLOGY 

muscle  and  a  sponge-work  of  fibrous  and  muscular  trabeculae, 
in  the  interstices  of  which  is  the  spleen  pulp.  The  branches 
of  the  splenic  artery  run  in  the  trabeculae,  and  twigs,  leaving 
these  trabeculse,  pass  out  and  are  covered  with  masses  of 
lymph  tissue  forming  the  Malpighian  corpuscles.  Beyond 
these,  the  vessels  open  into  a  series  of  complex  sinuses  from 
which  the  blood  is  collected  into  channels,  the  venous 
sinuses,  which  carry  it  back  to  branches  of  the  splenic  vein 
in  the  trabeculse.  The  pulp  is  thus  made  up  of  processes 
of  fibrous  tissue  and  of  a  set  of  branching  endothelial-like 
cells,  the  spaces  between  which  are  filled  with  blood.  It  is 
comparable  with  the  blood  sinuses  of  the  hsernolymph  glands. 

So  far  no  decrease  in  the  number  of  erythrocytes  in  the 
blood  leaving  the  organ  has  been  recorded.  In  the  cells  of 
the  spleen  pulp,  yellow  pigment  and  simple  iron  compounds 
are  frequently  seen,  indicating  that  haemoglobin  is  being 
broken  down.  But  the  idea  that  the  spleen  plays  an  im- 
portant part  in  the  actual  destruction  of  erythrocytes  seems 
to  be  negatived  by  the  fact  that,  when  blood  is  injected,  the 
cells  are  broken  down  no  faster  in  an  animal  with  the  spleen 
than  in  an  animal  from  which  the  spleen  has  been  removed. 
While  the  spleen  appears  to  have  no  action  in  killing  and 
destroying  erythrocytes,  its  cells  have  the  power  of  taking 
up  dead  and  disintegrating  erythrocytes  and  storing  the  iron 
for  future  use  in  the  body. 

The  non-striped  muscle  in  the  framework  of  the  spleen 
undergoes  rhythmic  contraction  and  relaxation,  and  the 
organ  thus  contracts  and  expands  at  regular  intervals  of 
about  a  minute. 

Lymph  Glands,  Hsemolymph  Glands,  and  Red  Marrow  of 
Bone. — When  erythrocytes  break  down  or  haemoglobin  is 
injected,  the  pigment  accumulates  in  these,  and  they  there- 
fore probably  act  as  the  graves  of  old  erythrocytes  in  the 
same  way  as  the  spleen. 

B.  LYMPH. 

1.  Characters  of  Lymph.— The  lymph  is  the  fluid  which 
plays  the  part  of  middleman  between  the  blood  and  the 
tissues.  It  fills  all  the  spaces  in  the  tissues  and  bathes  the 


THE   BLOOD  207 

individual  cell  elements.  These  spaces  in  the  tissues  open 
into  vessels — the  lymph  vessels — in  which  the  lymph  flows 
and  is  conducted  through  lymph  glands  and  back  to  the 
blood  through  the  thoracic  duct.  (See  Fig.  105,  p.  209.) 

Lymph  varies  in  character  according  to  the  situation  from 
which  it  is  taken  and  according  to  the  condition  of  the 
animal. 

Lymph  taken  from  the  lymph  spaces — e.g.  the  pericar- 
dium, pleura  or  peritoneum — is  a  clear,  straw-coloured  fluid. 
It  has  little  or  no  tendency  to  coagulate.  Microscopic 
examination  shows  that  it  contains  few  or  no  cells — any 
cells  which  may  exist  being  lymphocytes.  In  reaction  it 
is  alkaline.  The  specific  gravity  varies  according  to  its 
source. 

Apparently  the  cause  of  the  non-coagulation  of  such  lymph 
is  the  absence  of  cells  from  which  thrombin  may  be  set  free. 
If  blood  or  white  corpuscles  be  added  to  it,  a  loose  coagulum 
forms. 

If  the  lymph  be  taken  from  lymphatic  vessels  after  these 
have  passed  through  lymphatic  glands,  it  is  found  to  contain 
a  number  of  lymphocytes,  and  it  coagulates  readily. 

Chemically,  lymph  resembles  blood  plasma  in  which  the 
proteids  are  in  smaller  amount,  but  the  inorganic  salts  in 
the  same  proportion  as  in  the  blood.  The  amount  of  solids 
varies  in  lymph  from  different  organs. 

Lymph  of  Proteids. 

Limbs 2-3  per  cent. 

Intestines       ....         4-6         „ 
Liver 6-8         „ 

In  the  lymphatics  coming  from  the  alimentary  canal  during 
starvation,  the  lymph  has  the  characters  above  described. 
But  after  a  meal  it  has  a  milky  appearance  and  is  called 
chyle.  This  milky  appearance  is  due  to  the  presence  of  fats 
in  a  very  fine  state  of  division,  forming  what  is  called  the 
molecular  basis  of  the  chyle. 

Lymph  in  various  diseases  tends  to  accumulate  as  serous 
effusions  in  the  large  lymph  spaces — e.g.  the  pleura,  peri- 
toneum, pericardium — and  these  effusions  behave  differently 


208 


HUMAN   PHYSIOLOGY 


as  regards  coagulation.     The  following  table  helps  to  explain 
this  (S.  A.  is  Serum  Albumin,  S.  G.  Serum  Globulin) :— 


COAGULABILITY  OF  LYMPH,  SERUM,  AND  EFFUSIONS. 

Plasma  and 
Lymph. 

Serous  Effusion. 

Serum. 

Coag. 

Coag.  with 
Thrombi  ii. 

Uncoag. 

Uncoag. 

S.  A. 
S.  G. 
Fibrinogen. 
Fib.  Zym. 

S.  A. 
S.  G. 
Fibrinogen. 
Fib.  Zym. 

S.  A. 
S.  G. 
Fibrinogen. 

S.  A. 
S.  G. 

S.  A. 
S.  G. 

2.  Formation  of  Lymph. — Lymph  is  derived  partly  from 
the  blood  and  partly  from  the  tissues.  The  formation  of 
lymph  from  the  blood  depends  upon  the  permeability  of  the 
walls  of  the  capillaries  and  the  pressure  of  blood  in  the  blood 
vessels.  Thus,  although  the  pressure  in  the  blood  vessels  of 
the  limbs  is  much  higher  than  the  pressure  in  the  vessels  of 
the  liver,  hardly  any  lymph  is  usually  produced  in  the  former, 
while  very  large  quantities  are  produced  in  the  latter — 
apparently  because  of  the  small  permeability  of  the  limb 
capillaries  and  the  great  permeability  of  the  hepatic  capil- 
laries. The  permeability  may  be  increased  by  anything 
which  injures  the  capillary  wall.  Thus  the  injection  of  hot 
water  or  of  proteoses  at  once  leads  to  an  increased  flow  of 
lymph. 

But,  while  the  permeability  of  the  vessel  wall  is  the  most 
important  factor  controlling  lymph  formation,  any  increase 
of  the  intra-vascular  pressure  of  a  region  increases  the  flow 
of  lymph,  and  for  this  reason  any  obstruction  to  the  free  flow 
of  blood  from  a  part  leads  to  increased  lymph  production. 

That  lymph  is  also  formed  from  the  tissues  is  indicated 
by  the  fact  that  the  injection  of  substances  of  high  osmotic 
equivalent  into  the  blood — such  as  sugar  or  sulphate  of  soda 
— leads  to  a  flow  of  fluid  into  the  blood,  so  that  it  becomes 
diluted,  and  also  to  an  increased  flow  of  lymph,  and  this  in- 
crease of  water  in  both  can  be  explained  only  by  its  withdrawal 
from  the  tissues. 


SECTION   VII 

THE  CIRCULATION 

I.   General  Arrangement 

THE  arrangement  by  which  the  blood  and  lymph  are  dis- 
tributed to  the  tissues  may  be  compared  to  a  great  irrigation 
system. 

It  consists  of  a  central  force  pump — the  systemic  heart 
(Fig.  105,  S.H.) — from  which  pass  a  series  of  conducting  tubes 


LUNG 


CAP 


FIG.  105.— Scheme  of  the  Circulation.  S.H.,  Systemic  Heart  sending  Blood  to  the 
Capillaries  in  the  Tissues,  Cap.  The  Blood  brought  back  by  Veins  and 
the  exuded  Lymph  by  Lymphatics,  Ly.t  passing  through  glands  ;  Blood  sent 
to  the  Alimentary  Canal,  Al.C.,  and  from  that  to  the  Liver,  Liv.  ;  Blood  also 
sent  to  the  Kidneys,  Kid. ;  the  Blood  before  again  being  sent  to  the  body  is 
passed  through  the  Lungs  by  the  Pulmonic  Heart,  P.H. 

—the  arteries — leading  off  to  every  part  of  the  body,  and 
ending  in  innumerable  fine  irrigation  channels — the  capil- 
laries (Cap) — in  the  substance  of  the  tissues.  From  these,  a 
considerable  proportion  of  the  blood  constituents  is  passed 
into  the  spaces  between  the  cells  as  lymph.  From  these 
spaces  the  fluid  is  either  passed  back  into  the  capillaries,  or 
is  conducted  away  in  a  series  of  lymph  vessels,  which  carry 
it  through  lymph  glands  (Ly.\  from  which  it  gains  certain 

-9 


210  HUMAN   PHYSIOLOGY 

necessary  constituents,  and  finally  bring  it  back  to  the  central 
pump. 

The  fluid,  which  has  not  passed  out  of  the  capillaries  into 
the  tissues,  has  been  deprived  of  many  of  its  constituents, 
and  this  withdrawal  of  nutrient  material  by  the  tissues  is 
made  good  by  a  certain  quantity  of  the  blood  being  sent 
through  the  walls  of  the  stomach  and  intestine  (Al.C.),  in 
which  the  nutrient  material  of  the  food  is  taken  up  and  added 
to  the  blood  returning  to  the  heart.  At  the  same  time,  the 
waste  materials  added  to  the  blood  by  the  tissues  are  partly 
got  rid  of  by  a  certain  quantity  of  the  blood  being  sent 
through  the  liver  and  kidneys  (Liv.  and  Kid.). 

The  blood  is  then  poured  back,  not  at  once  into  the  great 
pump  which  sends  it  through  the  body,  but  into  a  subsidiary 
pump — the  pulmonic  heart  (P.H.) — by  which  it  is  pumped 
through  the  lungs,  there  to  obtain  a  fresh  supply  of  oxygen, 
and  to  get  rid  of  the  carbon  dioxide  excreted  into  it  by  the 
tissues.  Finally  the  blood,  with  its  fresh  supply  of  oxygen 
from  the  lungs,  and  of  nourishing  substances  from  the  ali- 
mentary canal,  is  poured  into  the  great  systemic  pump — the 
left  side  of  the  heart — again  to  be  distributed  to  the  tissues. 

Thus  the  circulation  is  arranged  so  that  the  blood,  ex- 
hausted of  its  nourishing  material  by  the  tissues,  is  replenished 
in  the  body  before  being  again  supplied  to  the  tissues. 

The  sectional  area  of  this  irrigation  system  varies  enor- 
mously. The  aorta  leaving  the  heart  has  a  comparatively 
small  channel.  If  all  the  arteries  of  the  size  of  the  radial 
were  cut  across  and  put  together,  their  sectional  area  would 
be  many  times  the  sectional  area  of  the  aorta.  And,  if  all 
the  capillary  vessels  were  cut  across  and  placed  together,  the 
sectional  area. would  be  about  700  times  that  of  the  aorta. 

From  the  capillaries,  the  sectional  area  of  the  veins  and 
lymphatics  steadily  diminishes  as  the  smaller  branches  join 
with  one  another  to  form  the  larger  veins  and  lymphatics : 
but,  even  at  the  entrance  to  the  heart,  the  sectional  area  of 
the  returning  tubes,  the  veins,  is  about  twice  as  great  as  that 
of  the  aorta  (Fig.  128,  p.  271). 

The  circulatory  system  may  thus  be  compared  to  a  stream 
which  flows  from  a  narrow  deep  channel,  the  aorta,  into  a 
gradually  broadening  bed,  the  greatest  breadth  of  the  channel 


THE   CIRCULATION  211 

being  reached  in  the  capillaries.     From  this  point  the  channel 
gradually  narrows  until  the  heart  is  reached. 

Hence  the  blood  stream  is  very  rapid  in  the  arteries  where 
the  channel  is  narrow,  and  very  sluggish  in  the  capillaries 
where  the  channel  is  wide,  so  that  in  them  plenty  of  time  is 
allowed  for  exchanges  between  the  blood  and  the  tissues. 


II.  The  Central  Pump— The  Heart. 

A.  Structure. — A  very  simple  form  of  heart  exists  in  the 
ascidians.  At  one  point  on  a  large  vessel  there  is  a  thicken- 
ing in  the  wall  composed  of  non-striped  muscular  fibres.  A 
contraction  is  seen  to  pass  from  one  end  of  this  to  the  other 
at  frequent  regular  intervals,  thus  forcing  the  fluid  through 
the  vessels.  The  embryonic  heart  in  man  has  a  similar 
structure. 

In  the  snail  and  cuttle-fish,  in  addition  to  the  contracting 
muscular  thickening,  there  is  also  a  thin- walled  receiving 
chamber  into  which  the  blood  flows  before  it  is  expelled 
onwards.  The  heart  is  thus  composed  of  two  chambers. 

1st.  A  receiving  chamber — the  auricle. 

2nd.  An  expelling  chamber — the  ventricle. 

In  fish  the  heart  has  a  similar  structure.  But  in  lung- 
bearing  animals  a  more  complex  arrangement  is  required, 
and  a  double  heart  is  found,  one  concerned  with  the  pro- 
pulsion of  blood  to  the  system  generally,  and  hence  called 
the  systemic  heart ;  one  propelling  blood  to  the  lungs,  and 
hence  called  the  pulmonic  heart.  In  mammals,  the  former 
chamber  is  on  the  left  side,  the  latter  on  the  right.  Each 
consists  of  a  receiving  and  expelling  chamber — an  auricle 
and  a  ventricle. 

The  walls  of  these  chambers  are  essentially  muscular ;  but 
this  muscular  layer,  or  myocardium,  lies  between  two  fibrous 
layers,  the  pericardium  and  the  endocardium. 

The  musculature  of  the  auricles  is  separate  from  that  of 
the  ventricles,  but  some  fibres  more  like  ordinary  visceral 
fibres  than  cardiac  muscles  extend  from  one  to  the  other. 
If  the  heart  is  boiled,  the  auricles,  the  aorta  and  the  pul- 
monary artery  may  be  separated  from  the  ventricles.  This 
is  because  boiling  converts  fibrous  tissue  to  gelatine  and 


212  HUMAN  PHYSIOLOGY 

dissolves  it,  and  it  is  by  white  fibrous  tissue  that  the  auricles 
and  great  arteries  are  attached  to  the  ventricles.  This  tissue 
is  arranged  in  three  rings,  one  encircling  the  opening  between 
the  right  auricle  and  ventricle  and  crescentic  in  shape ;  one, 
more  circular  in  shape,  encircling  in  common  the  left  auri- 
culo-ventricular  arid  the  aortic  orifice,  and  one  encircling 
the  pulmonary  opening.  The  auricles  are  attached  to  the 
auriculo-ventricular  rings  above,  the  ventricles  are  attached 
below,  while  the  valves  of  the  heart  are  also  connected  with 
them.  I  v 

The  muscular  fibres  of  the  auricles  are  arranged  in  two 
badly  defined  layers — . 

1st.  An  outer  layer  runnings,  horizontally  round  both 
auricles. 

2nd.  An  inner  layer  arching  over  each  auricle,  and  con- 
nected with  the  auriculo-ventricular  rings. 

Contraction  of  the  first  layer  diminishes  the  capacity  of 
the  auricles  from  side  to  side.  Contraction  of  the  second 
pulls  the  auricles  downwards  towards  the  ventricles,  and 
thus  diminishes  their  eapacity  from  above  downwards. 

The  peculiar  striped  muscle  fibres  of  the  auricular  walL 
extend  for  some  distance,  along  the  great  veins  which  open 
into  these  chambers. 

The  left  ventricle  forms  the  cylindrical  core  to  the  heart, 
and  the  right  ventricle  is  attached  along  one  side  of  it.  The 
septum  between  the  ventricles  is  essentially  the  right  wall  of 
the  left  ventricle,  and  it  bulges  into  the  right  ventricle  with 
a  double  convexity  from  above  downwards  and  from  before 
backwards  (Fig.  106). 

The  muscle  fibres  of  the  ventricle  are  arranged  essentially 
in  three  layers — 

1st.  The  outmost  layer  takes  origin  from  the  auriculo- 
ventricular  rings,  and  passes  downwards  and  to  the  left  till 
it  reaches  the  apex  of  the  heart.  Here  it  turns  inwards, 
forming  a  sort  of  vortex,  and  becomes  continuous  with  the 
inmost  layer. 

2nd.  The  middle  layer  is  composed  of  fibres  running  hori- 
zontally round  each  ventricle.  It  is  the  thickest  layer  of 
the  heart,  and  in  contracting  it  pulls  the  walls  of  the  ven- 
tricles towards  the  septum  ventriculi. 


THE   CIRCULATION 


213 


3rd.  The  inmost  layer  is  continuous  with  the  outmost  layer, 
as  it  turns  in  at  the  apex.  It  may  be  considered  as  composed 
of  two  parts — 

(a)  A  layer  of  fibres  running  longitudinally  along  the  in- 
side of  each  ventricle  from  the  apex  upwards  to  the  auriculo- 
ventricular  ring.  These  fibres  are  raised  into  fleshy  columns, 
the  columnse  carneae. 

(6)  A  set  of  fibres,  constituting  the  papillary  muscles 
(Fig.  107,  P.M.),  which,  taking  origin  from  the  apical  part  of 
the  ventricles,  extend  freely  upwards  to  terminate  in  a  series 


\FlG.    ],OG. — Cross  Section  thrjaugfr  the  Ventricles  of  thfi__He.act  looking  towards 

Auricles,  to  show  the  right  Ventricle  placed  on  the  Central  Core  of  the  left 
Ventricle.     The  cusps  of  the  Auriculo-ventricular  Valves  are  also  shown. 

of  tendinous  cords  (the  chordae  tendineae),  which  are  inserted 
partly  into  the  auriculo-ventricular  valves,  presently  to  be 
described,  and  partly  into  the  auriculo-ventricular  rings. 
The  papillary  muscles  are  merely  specially  modified  columme 
carnese.  In  many  cases,  actual  muscular  processes  extend 
from  the  apex  of  the  papillary  muscles  to  the  auriculo- 
ventricular  ring. 

In  the  left  ventricle  there  are  two  papillary  muscles,  or 
groups  of  papillary  muscles,  one  in  connection  with  the 
anterior  wall  of  the  ventricle,  called  the  anterior  muscle; 
and  one  in  connection  with  the  posterior  wall,  called  the 
posterior  muscle. 

In  the  right  ventricle  there  are — 1st.  One  or  more  small 


2i4  HUMAN  PHYSIOLOGY 

papillary  muscles  just  under  the  pulmonary  orifice,  and 
having  a  horizontal  direction,  their  apices  pointing  back- 
wards— the  superior  papillary  muscle  (Fig.  107,  S.P.M.). 

2nd.  A  large  papillary  muscle  taking  origin  from  the  mass 
of  fleshy  columns  at  the  apex  of  the  ventricle.  It  is  directed 
upwards.  It  may  be  called  the  anterior  papillary  muscle 
(AJP.M.). 

3rd.  One  or  more  papillary  muscles  of  varying  size  arising 
from  the  posterior  part  of  the  apical  portion  of  the  ventricle 
and  constituting  the  posterior  papillary  muscle  (P. P.M.). 

4th.  A  number  of  small  septal  papillary  muscles  arising 
from  the  septum. 

The  distribution  of  the  chorda?  from  these  muscles  will  be 
considered  in  connection  with  the  auriculo-ventricular  valves. 

In  contraction,  the  outmost  and  inmost  layers  of  the 
ventricles  tend  to  approximate  the  apex  to  the  base  of  the 
ventricles,  but  this  is  resisted  by  the  contracting  middle  layer. 
The  apex  tends  to  be  tilted  towards  the  right,  the  papillary 
muscles  shorten,  the  colummu  cameo;  by  their  shortening  and 
thickening  encroach  upon  the  ventricular  cavity,  and  help 
to  abolish  it,  while  the  auriculo-ventricular  rings  are  drawn 
downwards  and  inwards  towards  the  septum. 

The  endocardium  forms  a  continuous  fibrous  foyer,  lined 
by  endothelium,  extending  from  the  vessels  over  the  inner 
aspect  of  auricles  and  ventricles.  At  certain  points  flaps 
of  this  endocardium  are  developed  to  form  the  valves  of 
the  heart. 

In  the  heart,  valves  are  situated  at  the  entrance  to  and  at 
the  exit  from  the  expelling  cavities.     There  is  thus  on  each 
'  side  of  the  heart  a  valve  between  the  auricles  and  ventricles, 
\  and  a  valve  between  the  ventricles  and  the  great  arteries. 

Auriculo-ventricular  Valves, — On  each  side  of  the  heart 
the  auriculo-ventricular  valve  is  formed  by  flaps  of  endocar- 
dium, which  hang  downwards  from  the  auriculo-ventricular 
ring  like  a  funnel  into  the  ventricular  cavity,  and  are  attached 
to  the  apices  of  the  papillary  muscles  by  the  chordae  tendinese 
(Figs.  107  and  108). 

On  the  left  side,  of  the  heart  there  are  two  main  cusps, 
forming  the  mitral  valves  (Fig.  108)— 

1st.  An  anterior  or  right  cusp,  which  takes  origin  from, 


THE   CIRCULATION  215 

and  is  continuous  with,  the  right  posterior  wall  of  the  aorta, 
and  hangs  down  into  the  ventricle  between  the  aortic  and 
auriculo-ventricular  orifices,  thus  dividing  the  ventricle  into 
two  parts,  an  aortic  and  an  auricular  part.  This  cusp  is 
very  strong,  and  in  many  animals  bone  is  developed  in  it 
towards  its  base.  It  is  composed  of  dense  fibrous  tissue,  is 
smooth  on  both  sides,  and  the  chordae  are  inserted  chiefly 
along  its  edges. 

2nd.  The  posterior  or  left  cusp  takes  origin  from  the 
back  part  of  the  auriculo-ventricular  ring,  and  hangs  in  the 
ventricle  in  its  relaxed  state  against  the  posterior  and  left 
wall.  It  is  smaller  and  less  strongly  made  than  the  anterior 
cusp.  The  chordae  tendineae  are  not  only  inserted  into  its 
edge,  but  run  up  along  its  posterior  aspect  to  be  inserted 
into  the  auriculo-ventricular  ring,  and  they  thus  give  the 
posterior  aspect  of  the  cusp  a  rough  ridged  appearance. 

The  chordae  from  the  anterior  papillary  muscle  are  inserted 
into  the  anterior  or  left  edge  of  both  cusps,  those  from  the 
posterior  papillary  muscle  into  the  posterior  or  right  edges  of 
both  cusps.-  When  the  papillary  muscles  contract,  the  cusps 
are  thus  drawn  together.  The  edge  of  each  cusp  thins  out 
to  form  a  delicate  border,  which,  when  the  cusps  are  approxi- 
mated, completely  seals  the  aperture. 

On  the  right  side  of  the  heart  the  auriculo-ventricular 
orifice  is  separate  from  the  pulmonary  opening,  and  the  three 
cusps  of  the  tricuspid  valve  are  developed  in  connection 
with  the  crescentic  opening  from  the  auricle  (Fig.  106).  One 
rises  from  the  ring  above  the  septum,  and  hangs  down  into 
the  ventricle  upon  the  surface  of  the  septum.  This  cusp  is 
small,  thin,  and  delicate.  It  is  attached  by  its  lower  border 
to  the  septal  papillary  muscles.  The  chief  or  infundibular 
cusp  (Fig.  107,  I.C.}  rises  from  the  front  part  of  the  ring 
between  the  pulmonary  infundibulum  and  the  auriculo- 
ventricular  opening.  It  is  connected  by  its  anterior  border 
with  the  horizontal  fibres  from  the  superior  papillary  muscles, 
and  by  its  lower  and  inferior  border  by  the  chordae  from  the 
anterior  papillary  muscle.  When  these  two  sets  of  papillary 
muscles  contract,  this  cusp  is  drawn  flat  against  the  bulging 
septum. 

The  posterior  cusp  (P. C.)  takes  origin  from  the  posterior 


2l6 


HUMAN   PHYSIOLOGY 


and  outer  part  of  the  ring,  and  hangs  down  into  the  posterior 
part  of  the  ventricle.  It  is  connected  by  its  anterior  margin 
with  the  anterior  papillary  muscle  and  by  its  posterior  margin 
with  the  posterior  papillary  muscle.  Contraction  of  these 
muscles  will  therefore  approximate  its  anterior  edge  to  the 
infundibular  cusp,  its  posterior  edge  to  the  septal  cusp. 

In  both  the  infundibular  and  posterior  cusps  many  of  the 
chordae  pass  up  to  be  inserted  into  the  auriculo-ventricular 
ring. 

Semilunar  Valves. — The  valves,  situated  at  the  opening 


PA 


PPM 


FlO.  107. — The  Right  Ventricle  and  Tricuspid  Valve  to  show  relationship  of  the 
Papillary  Muscles  and  Chorda  Tendineae  to  the  Cusps  of  the  Valve.    (See  text.) 

of  the  ventricles  into  the  great  arteries,  are  also  formed  as 
special  developments  of  the  endocardium. 

Each  is  composed  of  three  half-moon-shaped  membranous 
pouches  attached  along  their  curved  margin  to  the  walls  of 
the  artery  and  upper  part  of  the  ventricle,  and  with  their 
concavities  directed  away  from  the  ventricle.  In  the  centre 
of  the  free  margin  is  a  fibrous  thickened  nodule,  the  corpus 
Arantii,  from  which  a  very  thin  piece  of  membrane,  the 
lunule,  extends  to  the  attached  margin  of  the  edges.  A 
pouch,  the  sinus  of  Valsalva,  lies  behind  each  cusp. 

The  arrangements  of  these  various  cusps  is  of  importance 
in  connection  with  their  action  (Fig.  108). 


THE   CIRCULATION  217 

Aortic  Valve. — The  anterior  cusp  is  largest,  and  lies  some- 
what deeper  in  the  heart  than  the  others.  At  each  side  it 
is  attached  to  the  aortic  wall,  but  below  it  is  attached  to  the 
upper  part  of  the  septum  ventriculi,  so  that  the  base  of  the 
sinus  of  Valsalva  is  formed  by  the  upper  part  of  the 
septum.  At  a  somewhat  higher  level  is  a  cusp  which 
is  partly  attached  to  the  upper  part  of  the  septum,  partly 


FIG.  108. — Vertical  Mesial  Section  through  Heart  to  show  Aortic  and  Mitral  Valves. 
R.V.,  Right  Ventricle;  L.V.,  Left < Ventricle  with  Papillary  Muscle;  L.A., 
Left  Auricle  ;  Ao. ,  Aorta  with  Anterior  Cusp. 

to  the  posterior  wall  of  the  aorta,  where  this  becomes  con- 
tinuous with  the  anterior  cusp  of  the  mitral.  The  third 
cusp  is  still  higher,  and  is  attached  to  the  aortic  wall 
where  it  becomes  continuous  with  the  anterior  cusp  of 
the  mitral. 

Pulmonary  Yalve. — The  posterior  cusp  is  mounted  on  the 
top  of  the  septum  ventriculi,  and  is  at  a  somewhat  lower 
level  than  the  other  two. 

Thus  in  each  valve  the  cusp  placed  lowest  is  mounted  on 


218 


HUMAN  PHYSIOLOGY 


a  muscular  cushion,  the  use  of  which  will   afterwards  be 
considered. 

Attachments  and  Relations  of  the  Heart  (Fig.  109). — 
The  heart  is  attached,  by  the  great  vessels  coming  from  it, 
to  the  posterior  wall  of  the  chest  at  the  level  of  the  5th 
to  the  8th  dorsal  vertebrae.  Its  plane  of  attachment  faces 
forwards  and  downwards.  From  this 
the  heart  projects  into  the  chest  as  a 
conical  mass  downwards,  forwards,  and 
to  the  left.  It  does  not  lie  at  right 
angles  to  its  plane  of  attachment,  but, 
when  not  contracting,  it  is  limp  and 
hangs  down,  as  shown  in  the  diagram 
(continuous  line).  Nor  can  it  assume 
a  position  at  right  angles  to  its  plane 
of  attachment  (dotted  line),  because  the 
front  part  of  the  heart  lies  against  the 
anterior  chest  wall  over  an  area  (the 
prsecordium)  bounded  to  the  right  by 
the  midsternal  line,  above  by  the  fourth 

FIG.  109. — Mesial  Section      i    rv      *u     i     i          i_      *.i  ^i     i    A.      -i 

through  the  Thorax  to    left  rib>  below  by  the   seventh  left  rib, 
show  the  attachment    and  to  the  left  by  a  vertical  line  inside 
the  nipple  line. 

From  the  oblique  position  of  the 
heart,  it  is  the  right  side — auricle  and  ventricle — which  is 
directed  forwards,  and  it  is  a  portion  of  the  right  ventricle 
which  lies  in  relationship  with  the  chest  wall.  The  rest  of 
the  organ  is  covered  by  the  lungs.  The  only  parts  of  the 
left  side  which  can  be  seen  from  the  front  are  the  tip  of 
the  left  auricular  appendix,  and  a  narrow  strip  of  the  left 
ventricle. 

Below  and  behind  the  heart  lies  upon  the  central  tendon 
of  the  diaphragm  to  which  the  pericardium  is  attached. 

All  round  it  are  the  lungs  completely  filling  up  the  rest  of 
the  thorax. 

The  heart  is  enclosed  in  a  strong  fibrous  bag,  the  Peri- 
cardium, which  supports  it  and  prevents  over-distension. 
When  fluid  accumulates  in  this  bag  the  auricles  are  pressed 
upon  and  the  flow  of  blood  into  them  is  impeded. 


and    relations    of    the 
Heart. 


THE   CIRCULATION 


219 


s/nus 


B.   Physiology  of  the  Heart. 

Cardiac  Cycle. — If  any  one  part  of  the  heart  of  a  frog  is 
watched  it  is  seen  to  undergo  contractions  and  relaxations  at 
regular  rhythmical  intervals. 

1.  General  Description. — In  the  frog  a  contraction,  starting 
from  the  openings  of  the  veins,  suddenly  involves  the  sinus 
venosus,  causing  it  to  become  smaller  and  paler.  This  con- 
traction is  rapid  and  of  short  duration,  and  is  followed  by  a 
relaxation,  the  cavity  again  regaining  its  former  size  and 
colour.  As  this  relaxation 
begins,  the  two  auricles 
are  suddenly  contracted 
and  pulled  downwards 
towards  the  ventricle  at 
the  same  time  becoming 
paler,  while  the  ven- 
tricle becomes  more  dis- 
tended and  of  a  deeper 
red.  The  rapid  brief  auri- 
cular contraction  now  gives 
place  to  relaxation,  and, 
just  as  this  begins,  the 
ventricle  is  seen  to  be- 
come smaller  and  paler, 
and  if  held  in  the  finger 
is  felt  to  become  firmer.  This  event  takes  place  more  slowly 
than  the  contraction  of  either  sinus  or  auricles.  The  chief 
change  in  the  ventricle  is  a  diminution  in  its  lateral  diameter, 
though  it  is  also  decreased  in  the  antero- posterior  and 
vertical  directions.  During  ventricular  contraction  the 
bulbus  is  seen  to  be  distended  and  to  become  of  a  darker 
colour.  The  ventricular  contraction  passes  off  suddenly,  the 
ventricle  again  becoming  larger  and  of  a  deep  red  colour. 
At  this  moment  the  bulbus  aortse  contracts  and  becomes  pale 
and  then  relaxes  before  the  next  ventricular  contraction. 

Each  chamber  of  the  heart  thus  passes  through  two 
phases — a  contraction  phase,  a  systole  of  short  duration, 
and  a  longer  relaxation  phase,  the  diastole.  And  the 


FIG.  110.— Scheme  of  the  Cardiac  Cycle  in 
the  Frog.  S.S.,  Sinus  Systole;  A.S., 
Auricular  Systole;  V.S.,  Ventricular 
Systole;  B.S.,  Bulbus  Systole;  P.,  Rest 
of  all  chambers. 


220  HUMAN   PHYSIOLOGY 

sequence  of  events  in  the  frog's  heart  might  be  schematically 
represented  us  in  Fig.  110. 

The  events  from  the  beginning  of  the  contraction  of  any 
cavity  to  the  beginning  of  the  next  contraction  of  that  cavity 
constitute  a  cardiac  cycle.  Usually  the  cardiac  cycle  is 
reckoned  from  the  beginning  of  the  systole  of  the  sinus 
(Practical  Physiology,  Chap.  IX.). 

In  the  mammalian  heart  the  rate  of  recurrence  of  the 
cardiac  contraction  varies  with  the  animal  examined.  In 
man  it  is  in  adult  life  about  72  per  minute.  Many  factors 
modify  the  rate  of  the  heart. 

1.  Period  of  Life. — The  following  table  shows  the  average 
rate  of  the  heart  at  different  ages : — 

Foetus          .        .        .         140  per  minute. 

Under  1  year      .         .         120 

1  to  3  years         .         .         100 

7  to  14  years       .         .  85 

21  to  61  years     .        .  70  to  75      „ 

Old  age       ...  75  to  80      „ 

2.  Period  of  the  Day. — The  pulse  is  generally  slowest  in 
the  early  morning,  and  quickest  in  the  evening. 

3.  Temperature  of  the  Body. — The  pulse  varies  with  the 
body  temperature,  generally  being  increased  about  10  beats 
with  each  degree  Fahr.  of  elevation  of  temperature. 

4.  Muscular  Exercise  increases  the  rate  of  the  heart,  first 
by  driving  the  blood  from  the  muscles  into  the  great  veins 
(p.  274),  and  second,  by  developing  toxic  substances  which 
act  directly  upon  the  heart. 

5.  Posture  has   also   an  important   influence.     Suddenly 
assuming  the  erect  position  accelerates  the  heart  by  caus- 
ing   the    blood    to    accumulate    in    the    abdominal   veins, 
and    thus    checking  its   transference   on   into   the   arteries 
(p.  276). 

6.  The  condition    of    the    central   nervous  system    may 
modify  the  rate  of  the  heart,  any  disturbance  accompanied 
by  emotional  changes  either  accelerating  or  retarding  the 
rate. 

7.  Stimulation  of  certain  nerves — especially  those  of  the 


THE   CIRCULATION 


221 


abdomen — tend  to  cause  a  retardation  in  the  rate  of  the 
heart. 

The  sequence  of  events  making  up  the  cardiac  cycle  is 
simpler  than  in  the  frog. 

The  contraction  starts  in  the  great  veins  which  enter  the 
auricles,  and  spreads  down  along  them  to  these  chambers. 
This  corresponds  to  the  contraction  of  the  sinus  in  the  frog's 
heart.  It  is  followed  by  a  short  sharp  contraction  of  the 
auricles,  which  become  smaller  in  all  directions  and  seem  to 
be  pulled  down  towards  the  ventricles.  The  contraction  of 
the  auricles  in  mammals  is  not  accompanied  by  so  marked  a 
dilatation  of  the  ventricles  as  in  the  frog. 

After  the  auricles  have  fully  contracted,  the  contraction  of 
the  ventricles  begins,  and  immediately  the  auricles  relax  and 
resume  their  original  size. 

The  ventricular  contraction  develops  suddenly,  lasts  for 
some  time,  and  then  suddenly  passes  of. 

The  contraction  of  the  ventricle  is  followed  by  a  period 
during  which  both  auricles  and  ventricles  remain  relaxed. 
This  is  called  the  pause  of  the  cardiac  cycle. 

The  cardiac  cycle  in  mammals  may  be  represented  as 
in  Fig.  Ill  :— 

2.  Duration  of  the  Phases. — Ventricular  systole  lasts  three 
times  as  long  as  auricular  systole ;  the  auricles  contract  for 
about  O'l  of  a  second, 
the  ventricles  for  0*3 
of  a  second. 

The  duration  of 
these  two  phases  in 
relationship  to  the 
pause  varies  very 

greatly.  Whatever    FIG.    111. — Scheme    of    the  Cardiac    Cycle  in   the 

<-"U^     M«4^     ^f  Human  Heart.     A.S. ,  Auricular  Systole ;   V.S., 

may    be     the     rate    Of  Ventricular  Systole;  P.,  Pause. 

the  heart,  the  auri- 
cular and  ventricular  systoles  do  not  vary,  but  in  a  rapidly 
acting  heart  the  pause  is  short,  in  a  slowly  acting  heart  it 
is  long.  Taking  the  ordinary  heart  rate  of  72  per  minute, 
the  auricular  systole  lasts  for  |th  of  the  whole  cardiac 
cycle,  the  ventricular  for  f  ths,  and  the  pause  for  f  ths. 


AWICLES 


VENTRICLES 


222  HUMAN  PHYSIOLOGY 

3.  Changes  in  Shape  of  the  Chambers. 

1.  Auricles. — These  simply  become  smaller  in  all  direc- 
tions during  systole. 

2.  Ventricles. — The    changes    in    the    diameters    of    the 
ventricles  may  be  studied  by  fixing  them  in   the  various 
phases  of  contraction  and  measuring  the  alterations  in  the 
various  diameters, 

The  shape  in  diastole  may  be  investigated  after  death- 
stiffening  has  passed  off  and  has  left  the  walls  relaxed.  The 
condition  at  the  end  of  systole  may  be  studied  by  rapidly 
excising  it  while  it  is  still  beating  and  plunging  it  in  some 
hot  solution  to  fix  its  contraction. 

The  condition  at  the  beginning  of  systole,  before  the  blood 
has  left  the  ventricles,  may  be  studied  by  applying  a  ligature 
round  the  great  vessels  and  then  plunging  the  heart  in  a  hot 
solution  to  cause  it  to  contract  round  the  contained  blood 
which  cannot  escape. 

Measurements  of  hearts  so  fixed  show  that  at  the  begin- 
ning of  contraction  the  antero-posterior  diameter  is  increased, 
while  the  lateral  diameter  is  diminished.  In  contracting, 
the  lateral  walls  appear  to  be  pulled  towards  the  septum — 
the  increase  in  the  antero-posterior  diameter  being  largely  due 
to  the  blood  in  the  right  ventricle  pressing  on  and  pushing 
forward  the  thin  wall  of  the  conus. 

As  the  ventricles  drive  out  their  blood,  both  antero- 
posterior  and  lateral  diameters  are  diminished — but  the 
diminution  in  the  lateral  direction  is  the  more  marked. 

There  is  no  great  shortening  in  the  long  axis  of  the  heart. 
Although  the  contraction  of  the  longitudinal  fibres  tends  to 
approximate  base  and  apex,  this  is  in  part  prevented  by  the 
contraction  of  the  circular  fibres. 

4.  Change  in  Position  of  the  Heart. — During  contraction 
the  heart  undergoes,  or  attempts  to  undergo,  a  change  in 
position.     In  the  relaxed  condition  it  hangs  downwards  and 
to  the  left  from  its  plane  of  attachment,  but  when  it  becomes 
rigid  in  ventricular  contraction  it  tends  to  take  a  position  at 
right  angles  to  its  base — Cor  sese  erigere,  as  Harvey  describes 
the  movement.     Since  the  apex  and  front  wall  are  in  contact 
with  the  chest,  the  result  of  this  movement  is  to  press  the 


THE  CIRCULATION  223 

heart  more  forcibly  against  the  chest  wall.  This  gives  rise 
to  the  cardiac  impulse  which  is  felt  with  each  ventricular 
systole  over  the  prsecordium  (Fig.  109). 

If  the  chest  is  opened  and  the  animal  placed  on  its  back 
this  elevation  of  the  apex  is  readily  seen.  If  the  animal  is 
placed  on  its  belly,  so  that  the  heart  when  relaxed  hangs 
forwards,  the  apex  is  tilted  back  during  contraction. 

Since  the  apex  is  twisted  to  the  left,  the  movement  of  the 
ventricle  is  not  simply  directly  forward,  but  also  from  left 
to  right.  This  tilting  of  the  apex  from  left  to  right  is 
further  favoured  by  the  direction  of  the  muscular  fibres  of 
the  ventricles  which  pass  from  the  auriculo- ventricular  rings 
downwards  and  to  the  left. 

The  increased  thickness  of  the  heart  from  before  back- 
wards also  assists,  to  some  extent,  in  the  production  of  the 
impulse. 

The  study  of  the  position  and  characters  of  the  cardiac 
impulse  is  of  great  importance  in  medicine. 

The  position  is  determined  by  the  relationship  of  the  heart 
to  the  anterior  chest  wall  and  to  the  lungs.  The  boundaries 
of  the  part  of  the  heart  lying  in  relationship  to  the  chest 
wall  have  been  already  defined  (p.  218),  and  it  is  at  the  outer 
and  lower  part  of  this  area,  a  region  bounded  above  by  the 
fifth  rib,  below  by  the  sixth  rib,  outside  by  a  line  drawn  verti- 
cally through  the  nipple,  and  inside  by  a  line  drawn  vertically 
midway  between  the  nipple  and  the  left  edge  of  the  sternum, 
that  the  cardiac  impulse  is  felt.  Normally  it  does  not  extend 
outwards  beyond  the  nipple  line,  but  frequently  when  the 
left  lung  is  voluminous  the  impulse  only  extends  out  to  an 
inch  or  so  inside  of  the  nipple  line.  In  children,  on  account 
of  the  size  of  the  liver,  it  is  often  felt  between  the  fourth  and 
fifth  ribs.  This  impulse  is  often  called  the  apex  beat — but  it 
is  not  the  apex  which  presses  on  the  chest  wall  but  a  part  of 
the  front  of  the  right  ventricle. 

In  character  it  is  felt  as  a  forward  impulse  of  the  tissue 
occupying  the  fifth  interspace,  which  develops  suddenly, 
persists  for  a  short  period,  and  then  suddenly  disappears. 
In  many  forms  of  heart  disease  its  character  is  markedly 
altered. 

The    cardiac    impulse    may  be   recorded   graphically   by 


224 


HUMAN   PHYSIOLOGY 


means  of  any  of  the  various  forms  of  cardiograph,  one  of 
the  simplest  consisting  of  a  receiving  and  recording  tam- 
bour connected  by  means  of  a  tube  (Fig.  112).  (Practical 
Physiology,  Chap.  XL) 


FlO.  112. — Cardiograph  consisting  of  a  Receiving  Tambour,  with  a  button  on  the 
Membrane  which  is  placed  upon  the  Cardiac  Impulse,  and  a  Recording 
Tambour  connected  with  a  Lever. 

The  form  of  the  trace  varies  according  to  the  part  of  the 
heart  upon  which  the  button  is  placed,  but  it  has  the  char- 
acter shown  in  Fig.  113 
if  the  button  is  upon 
the  cardiac  impulse. 

At  the  moment  of 
ventricular  systole  the 
lever  is  suddenly 
thrown  up  to  a  certain 
level  (a  to  b).  From 
this  point  it  suddenly  falls  slightly  (6  to  c),  but  is  maintained 
during  the  ventricular  systole  above  the  abscissa  (c  to  d). 
At  the  end  of  the  ventricular  systole,  as  the  heart  falls 
away  from  the  chest  wall,  the  lever  falls  to  its  original  level 
(d  to  e).  In  many  tracings  a  small  rise  of  the  lever  may 


FIG.  113.— Cardiographic  Trace,     a  to  d, 
Ventricular  Contraction. 


THE   CIRCULATION  225 

be  seen  just  before  the  great  upstroke.  This  corresponds 
to  the  contraction  of  the  auricles. 

In  various  diseases  of  the  heart  the  cardiogram  is  mate- 
rially modified.  Hence  it  is  important  to  have  a  clear 
conception  of  the  various  parts  of  the  trace. 

The  elucidation  of  the  various  parts  of  the  cardiogram  is 
only  possible  after  careful  study  of  the  other  changes  in  the 
heart  during  the  cycle. 

5.  Changes  in  the  Intracardiac  Pressure. 

These  can  be  studied  only  in  the  lower  animals. 

The  most  common  way  of  determining  the  pressure  in  a 
cavity  is  to  connect  it  to  a  vertical  tube  and  to  see  to  what 
height  the  fluid  in  the  cavity  is  raised.  If  such  a  method  is 
applied  to  the  heart,  the  blood  in  the  tube  undergoes  such 
sudden  and  enormous  changes  in  level  that  it  is  impossible 
to  get  accurate  results. 

The  same  objection  applies  to  the  method  of  connecting 
the  heart  with  a  U  tube  filled  with  mercury.  When  this  is 
done  the  changes  in  pressure  are  so  sudden  and  so  extensive 
that  the  mercury  cannot  respond  to  them  on  account  of  its 
inertia. 

Various  means  of  obviating  these  difficulties  have  been 
devised.  One  of  the  best  is  to  allow  the  changes  of  pressure 
to  act  upon  a  small  elastic  membrane  tested  against  known 
pressures.  A  tube  is  thrust  through  the  wall  of  the  heart 
and  connected  with  a  tambour  covered  by  a  membrane  to 
which  a  lever  is  attached. 

A.  Pressure  in  the   Great    Veins  (small  dotted   line   in 
Fig.  114). — When  the  auricles  contract  the  flow  of  blood  from 
the  great  veins  into  these  chambers  is  arrested,  and,  as  a 
result,  the  pressure  in  the  veins  rises.     As  the  auricles  relax 
the  blood  is  sucked  from  the  veins  and  the  pressure  falls,  but, 
as  the  auricles  fill  up,  it  again  rises.     When  the  ventricles 
relax  and  suck  blood  from  the  auricles,  blood  again  flows  in 
from  the  great  veins  and  the  pressure  falls,  again  to  rise  as 
the  auricles  and  veins  are  both  filled  up,  towards  the  end  of 
the  pause. 

B.  Pressure  in  the  Auricles   (dash   line   in    Fig.   114). 

At  the  moment  of  auricular  contraction  there  is  a  marked 

15 


226 


HUMAN  PHYSIOLOGY 


rise  in  the  intra-auricular  pressure.      When  the  auricular 
systole  stops,  the  pressure  falls  rapidly,  reaching  its  lowest 


A  S.          V.  S. 


AS. 


PRESSUHE  IN 


Artery. 

t 

Auricle. 
Great  Veins 
Ventricle.  * 


FLOW    of  BLOOD 
from 

1.  Great  Veins  to  Auricles. 


2.  Auricles  to  Ventricles- 


S.  Ventricles  to  Arteries. 


CLOSURE    o^ 

1.  Aunculo-Ventricular  Valve. 

2.  Semilunar  Valves. 
SOUNDS    of  HEART. 

CARDIAC     IMPULSE. 


FIG.  114. — Diagram  to  show  the  relationship  of  the  events  in  the  Cardiac  Cycle  to 
one  another.     A.S.,  Auricular  Systole ;  V.S.,  Ventricular  Systole ;  P.,  Pause. 

level  when  the  ventricles  are  throwing  their  blood  into  the 
arteries.     Sometimes  there  is  an  interruption  to  this  descent, 


THE   CIRCULATION  227 

apparently  synchronous  with  the  closure  of  the  auriculo- 
ventricular  valves.  From  this  point  the  pressure  in  the 
auricles  rises  until  the  moment  when  the  ventricles  relax, 
when  another  fall  in  the  pressure  is  observed.  The  pressure 
remains  about  constant  from  this  point  until  the  next 
auricular  contraction. 

C.  Pressure  in  the  Ventricles  (continuous  line  in  Fig.  114). — 
The  intra-ventricular  pressure  suddenly  rises  at  the  moment 
of  ventricular  systole  to  reach  its  maximum.     From  this  it 
falls,  but  4, he  fall  is  gradual,  and  is  interrupted  by  a  more  or 
less  well-marked  period  during  which  the  pressure  remains 
constant.     As  the  ventricles  relax  the  pressure  suddenly  falls 
to  below  zero,  and  then  rises  to  a  little  above  zero,  at  which  it 
is  maintained  until  the  next  ventricular  systole.     The  dia- 
stolic  expansion  of  the  ventricle  is  in  part  due  to  the  elasticity 
of  the  muscular  wall,  and  in  part  to  the  filling  of  the  coronary 
arteries  which  takes  place  only  as  the  muscular  fibres  relax. 

D.  Pressure  in  the  Arteries  (dot-dash  line  in  Fig.  114). — 
There  is  a  sudden  rise  in  the  aortic  pressure  as  the  blood 
rushes  out  of  the  ventricles.     The  pressure  then  falls,  but  the 
fall  is  not  steady.     Often  it  is  interrupted  by  a  more  or  less 
marked  increase   corresponding   to   the   later  part   of    the 
ventricular    contraction.      At   the   moment   of    ventricular 
diastole,  the  fall  is  very  sharp  and  is  interrupted  by  a  well- 
marked  and  sharp  rise.     Following  this  the  fall  is  continuous 
till  the  next  systolic  elevation. 

In  the  dog  the  extent  of  variation  of  the  pressure  in 
auricles  and  ventricles  is  roughly  as  follows — measured  in 
millimetres  of  Hg — 

Left  Right  Eight 

Ventricle.  Ventricle.  Auricle. 

Maximum          .     +140  +60  +30 

Minimum  .     —   30  —15  —   7 

These  changes  in  the  pressure  in  the  different  chambers 
are  due — 

1st  To  the  alternate  systole  and  diastole  of  the  chambers, 
the  first  raising,  the  second  lowering  the  pressure  in  the 
chambers. 

2nd.  To  the  action  of  the  valves. 


228 


HUMAN  PHYSIOLOGY 


6.  Action  of  the  Valves  of  the  Heart. 

A.  Auriculo-ventricular  (Fig.  115). — These  valves  have 
already  been  described  as  funnel-like  prolongations  of  the 
auricles  into  the  ventricles.  They  are  firmly  held  down  in 
the  ventricular  cavity  by  the  chordae  tendineae.  When  the 
ventricle  contracts  the  papillary  muscles  pull  the  cusps  of  the 
valves  together  and  thus  occlude  the  opening  between  auricles 
and  ventricles.  The  cusps  are  further  pressed  face  to  face  by 
the  increasing  pressure  in  the  ventricles,  and  they  may  become 
convex  towards  the  auricles.  They  thus  form  a  central  core 
around  and  upon  which  the  ventricles  contract. 

On  the  -left  side  of  the  heart  the  strong  anterior  cusp  of 
the  mitral  valve  does  not  materially  shift  its  position.  It 


FlG.  115. — State  of  the  various  parts  of  the  Heart  throughout  the  Cardiac  Cycle. 
1,  Auricular  Systole ;  2,  Beginning  of  Ventricular  Systole  (latent  period) ; 
3,  Period  of  Outflow  from  the  Ventricle ;  4,  Period  of  Residual  Contraction  ; 
5,  Beginning  of  Ventricular  Diastole. 

may  be  somewhat  pulled  backwards  and  to  the  left.     The 
posterior  cusp  is  pulled  forwards  against  the  anterior. 

On  the  right  side  the  infundibular  cusp  of  the  tricuspid 
valve  is  stretched  between  the  superior  and  inferior  papillary 
muscles,  and  is  thus  pulled  towards  the  bulging  septum, 
against  which  it  is  pressed  by  the  increasing  pressure  inside 
the  ventricles.  The  posterior  cusp  has  its  anterior  margin 
pulled  forward  and  its  posterior  margin  backwards,  and  is 
thus  also  pulled  toward  the  septum.  The  septal  cusp 
remains  against  the  septum.  The  greater  the  pressure  in 
the  ventricle  the  more  firmly  are  these  cusps  pressed  against 
one  another  or  against  the  septum,  and  the  more  completely 
is  the  orifice  between  the  auricle  and  the  ventricle  closed. 
On  the  right  side  of  the  heart  other  factors  play  an  impor- 
tant part  in  occluding  the  orifice ;  the  muscular  fibres  which 


THE  CIRCULATION  229 

surround  the  auriculo-ventricular  opening  contract,  while  the 
papillary  muscles  pull  the  auriculo-ventricular  ring  down- 
wards and  inwards  through  the  chordae  which  are  inserted 
into  it. 

Nevertheless  the  occlusion  of  this  orifice  is  apt  to  be  in- 
complete when  the  right  side  of  the  heart  becomes  in  the 
least  over-distended,  giving  rise  to  a  safety  valve  action  for 
the  right  ventricle. 

The  auriculo-ventricular  valves  are  open  during  the  whole 
of  the  cardiac  cycle,  except  during  the  ventricular  systole 
(Fig.  114). 

B.  Semilunar  Valves.  —  Before  the  ventricles  contract 
these  valves  are  closed  and  the  various  segments  pressed 
together  by  the  high  pressure  of  blood  in  the  aorta. 

As  the  ventricles  contract  the  pressure  in  them  rises,  until 
the  intra-ventricular  pressure  becomes  greater  than  the  pres- 
sure in  the  arteries.  Instantly  the  cusps  of  the  valves  are 
thrown  back  and  remain  thus  until  the  blood  gushes  out. 
When  the  outflow  of  blood  is  completed,  the  cusps  are  again 
approximated  by  the  pressure  of  blood  in  the  arteries.  As 
relaxation  of  the  ventricles  occurs,  the  intra-ventricular 
pressure  becomes  suddenly  very  low,  and  the  high  pressure 
of  the  blood  in  the  arteries  at  once  falls  upon  the  upper 
surfaces  of  the  cusps  which  are  thus  forced  downwards  and 
completely  prevent  any  backflow  of  blood. 

The  prejudicial  effect  of  too  great  pressure  upon  these 
cusps  is  obviated  by  the  lower  cusp  being  mounted  on 
the  top  of  the  muscular  septum  upon  which  the  pressure 
comes — the  other  cusps  shutting  down  upon  this  one 
(Fig.  108). 

7.  Flow  of  Blood  through  the  Heart. — The  circulation  of 
blood  through  the  heart  depends  upon  these  differences  of 
pressure  in  the  different  chambers. 

A  fluid  always  flows  from  a  point  of  high  pressure  to  a 
point  of  lower  pressure.  We  may  then  consider  the  flow 

A.  From  Great  Veins  into  Auricles. — This  occurs  when 
the  pressure  in  the  great  veins  is  greater  than  the  pressure  in 
the  auricles  (Fig.  114). 

The  pressure  in  the  auricles  is  lowest  at  the  moment  of 


230  HUMAN  PHYSIOLOGY 

their  diastole.  At  this  time  there  is  therefore  a  great  fl«>\v 
of  blood  into  them,  but  gradually  this  becomes  less  and  less, 
until,  when  the  ventricles  dilate,  another  fall  in  the  auricular 
pressure  takes  place  and  another  rush  of  blood  from  the  gr»  at 
veins  occurs.  Gradually  this  diminishes,  and  by  the  time 
that  the  auricles  contract  the  flow  from  the  great  veins  has 
stopped. 

The  contraction  of  the  mouths  of  the  great  veins,  which 
precedes  the  auricular  systole,  drives  blood  from  the  veins 
into  the  auricles,  and,  as  these  enter  into  contraction,  no  flow 
from  the  veins  can  occur,  and  no  back  flow  from  the  auricles 
is  possible  (Fig.  114). 

B.  From  Auricles  to  Ventricles. — As  the  ventricles  dilate, 
a  very  low  pressure  develops  in  them,  and  hence  a  great  rush 
of  blood  occurs  from  the  auricles.     During  the  passive  stage 
of  ventricular  diastole,  the  intra- ventricular  pressure  becomes 
nearly  the  same  as  the  auricular,  and  the  flow  diminishes  or 
may  stop.     When  the  auricles  contract  a  higher  pressure  is 
developed,  and  a  fresh  flow  of  blood  occurs  into  the  ventricles. 
When  the  ventricles  contract  the  auriculo-ventricular  valves 
are  closed,  and  all  flow  of  blood  from  the  auricles  is  stopped 
(Fig.  114). 

C.  From   Ventricles  to  Arteries.  —  When   the   ventricles 
begin  to  contract  the  intra-ventricular  pressure  is  low,  while 
the  pressure  in  the  arteries  is  high  and  keeps  the  semilunar 
valves  shut.     As  ventricular  systole  goes  on  the  intra-ven- 
tricular pressure  rises  until  after  about  0-03  of  a  second  it 
becomes  higher  than  the  arterial  pressure  (Latent  Period). 
Immediately  the  semilunar  valves  are  forced  open  and  a  rush 
of  blood  occurs  from  the  ventricles  (Period  of  Overflow). 
This  usually  lasts  less  than  O2  second.     If  the  ventricles  are 
acting  powerfully,  and  if  the  pressure  in  the  arteries  does  not 
offer  a  great  resistance  to  the  entrance  of  blood,  then  the 
ventricles  rapidly  empty  themselves  into  the  arteries.     If  the 
heart,  however,  is  not  acting  forcibly,  or  if  the  arterial  pressure 
offers  a  great  resistance  to  the  entrance  of  blood,  then  the 
outflow  is  slow  and  more  continued.     In  the  former  case  we 
get  a  trace  of  the  intra-ventricular  pressure  like  Fig.  121,  a, 
p.  252,  with  a  well-marked  Period  of  Residual  Contraction, 
and  in  the  latter  case  the  trace  is  like  Fig.  121,  b.     It  is  not 


THE  CIRCULATION  231 

so  much  the  absolute  force  of  the  cardiac  contraction  or  the 
absolute  intra-arterial  pressure  which  governs  this,  as  the 
relationship  of  the  one  to  the  other.  The  heart  may  not  be 
acting  very  forcibly,  but  still  if  the  pressure  in  the  arteries  is 
low  its  action  may  be  'relatively  strong. 

The  Coronary  Arteries,  unlike  all  the  other  arteries,  are 
filled  during  ventricular  diastole.  During  systole  they  are 
compressed  by  the  contracting  muscle  of  the  heart,  and  it 
is  only  when  that  compression  is  removed  in  diastole  that 
blood  rushes  into  them  and  helps  to  dilate  the  ventricles. 

The  interpretation  of  the  various  details  of  the  Cardiogram 
is  now  rendered  more  easy.  The  ventricles,  still  full  of 
blood,  are  suddenly  pressed  against  the  chest  wall.  As  the 
blood  escapes  into  the  arteries  they  press  with  less  force,  and 
hence  the  sudden  slight  downstroke  (Fig.  113,  b  to  c).  But  so 
long  as  the  ventricles  are  contracted  the  apex  is  kept  tilted 
forward,  and  hence  the  horizontal  plateau  is  maintained 
(c  to  d)  and  disappears  as  the  ventricles  relax  (e). 

8.  Sounds  of  the  Heart. — On  listening  in  the  region  of  the 
heart,  a  pair  of  sounds  may  be  heard  with  each  cardiac  cycle, 
followed  by  a  somewhat  prolonged  silence.  These  are  known 
respectively  as  the  First  and  Second  Sounds  of  the  Heart 
(Fig.  114).  (Practical  Physiology,  Chap.  XL) 

By  placing  a  finger  on  the  cardiac  impulse  while  listening 
to  these  sounds  it  is  easy  to  determine  that  the  first  sound 
occurs  synchronously  with  the  cardiac  impulse — i.e.  syn- 
chronously with  the  ventricular  contraction. 

It  develops  suddenly,  and  more  slowly  dies  away.  In 
character  it  is  dull  and  rumbling,  and  may  be  imitated  by 
pronouncing  the  syllable  lub.  In  pitch  it  is  lower  than  the 
second  sound. 

The  second  sound  is  heard  at  the  moment  of  ventricular 
diastole.  Its  exact  time  in  the  cardiac  cycle  has  been  deter- 
mined by  recording  it  on  the  cardiac  tracing  by  means  of  a 
microphone.  It  develops  suddenly  and  dies  away  suddenly. 
It  is  a  clearer,  sharper,  and  higher  pitched  sound  than  the 
first.  It  may  be  imitated  by  pronouncing  the  syllable  dupp. 

According  to  the  part  of  the  chest  upon  which  the  ear  is 


232  HUMAN  PHYSIOLOGY 

placed,  these  sounds  vary  in  intensity.  Over  the  apical 
region  the  first  sound  is  louder  and  more  accentuated  ;  over 
the  base  the  second  sound  is  more  distinctly  heard. 

The  Cause  of  the  Second  Sound  is  simple.  At  the  moment 
of  ventricular  diastole,  when  this  sound  develops,  fehe  only 
occurrence  which  is  capable  of  producing  a  sound  is  the 
sudden  stretching  of  the  semilunar  valves  by  the  high 
arterial  pressure  above  them  and  the  low  intra-ventricular 
pressure  below  them.  The  high  arterial  pressure  comes  on 
them  suddenly  like  the  blow  of  a  drum-stick  on  a  drum- 
head, and,  by  setting  the  valves  in  vibration,  produces  the 
sound. 

Aortic  and  Pulmonary  Areas. — The  second  sound  has  thus 
a  dual  origin — from  the  aortic  valve  and  from  the  pulmonary 
valve ;  and  it  is  possible  by  listening  in  suitable  positions  to 
distinguish  the  nature  of  each  of  these. 

The  aortic  valve  is  placed  behind  the  sternum  at  the  level 
of  the  lower  border  of  the  third  costal  cartilage.  But  it  is 
deeply  situated.  The  aorta,  passing  upwards  and  forwards, 
lies  in  close  relationship  to  the  chest  wall  at  the  junction  of 
the  right  side  of  the  sternum  and  the  right  second  costal 
cartilage.  The  sound  produced  by  the  valve  is  conducted 
up  the  aorta,  and  may  best  be  heard  in  this  "  aortic 
area." 

On  the  other  hand,  the  pulmonary  valve  lies  in  close  re- 
lationship to  the  anterior  chest  wall — being  covered  only  by 
the  anterior  border  of  the  left  lung — close  to  the  edge  of  the 
sternum  in  the  second  left  interspace.  The  pulmonary 
element  of  the  second  sound  may  best  be  heard  here. 

The  Cause  of  the  First  Sound  is  by  no  means  so  simple. 
When  it  is  heard,  two  changes  are  taking  place  in  the  heart, 
either  of  which  would  produce  a  sound. 

1st.  The  muscular  wall  of  the  ventricles  is  contracting. 

2nd.  The  auriculo-ventricular  valves  are  being  closed  and 
subjected  on  the  one  side  to  the  high  ventricular  pressure, 
and  on  the  other  to  the  low  auricular  pressure. 

That  the  first  factor  plays  an  important  part  hi  the  pro- 
duction of  the  first  sound  is  proved  by  rapidly  cutting  out 
the  heart  of  an  animal,  and  while  it  is  still  beating — but 
without  any  blood  passing  through  it  to  stretch  the  valves — 


THE   CIRCULATION  233 

listening  to  the  organ  with  a  stethoscope.  With  each  beat 
the  lub  sound  is  distinctly  heard. 

Apparently  the  wave  of  contraction,  passing  along  the 
muscular  fibres  of  the  heart,  sets  up  vibrations,  and  when 
these  are  conducted  to  the  ear  the  external  meatus  picks  out 
the  vibration  corresponding  to  its  fundamental  note,  and  thus 
produces  the  characters  of  the  sound. 

2nd.  The  stretching  of  the  auriculo-ventricular  valves  also 
plays  a  part.  If  the  valves  be  destroyed  or  diseased  the 
characters  of  the  first  sound  are  materially  altered,  or  the 
sound  may  be  entirely  masked  by  a  continuous  musical 
sound — a  murmur.  Again,  it  has  been  maintained  that  a 
trained  ear  can  pick  out  in  the  first  sound  the  note  corre- 
sponding to  the  valvular  vibration. 

The  idea  that  the  impulse  of  the  heart  against  the  chest 
wall  plays  a  part  in  the  production  of  this  sound  is  based  upon 
the  fallacious  idea  that  the  heart  "  hits  "  the  chest  wall.  All 
that  it  does  is  to  press  more  firmly  against  it. 

Mitral  and  Tricuspid  Areas. — On  account  of  the  part 
played  by  the  valves  in  the  production  of  the  first  sound  it 
may  be  considered  to  be  double  in  nature — partly  due  to 
the  mitral  valve,  partly  to  the  tricuspid.  The  mitral  valve 
element  may  best  be  heard  not  over  the  area  of  the  mitral 
valve — which  lies  very  deep  in  the  thorax — but  over  the 
apex  of  the  heart,  as  at  this  situation  the  left  ventricle,  in 
wnich  the  valve  lies,  comes  nearest  to  the  thoracic  wall  and 
conducts  the  sound  thither.  The  tricuspid  element  may  be 
best  heard  over  the  area  of  the  valve,  and  in  listening  to  it 
it  is  usual  to  go  to  the  right  extremity  of  the  area  in  order 
as  far  as  possible  to  eliminate  the  mitral  sound.  The  best 
situation  to  select  is  at  the  junction  of  the  fifth  right  costal 
cartilage  with  the  sternum. 

Cardiac  Murmurs. — When  these  valves  are  diseased  and 
fail  to  act  properly,  certain  continuous  sounds  called  cardiac 
murmurs  are  heard. 

These  owe  their  origin  to  the  fact  that,  while  a  current  of 
fluid  passing  along  a  tube  of  fairly  uniform  calibre  is  not 
thrown  into  vibrations  and  therefore  produces  no  sound, 
when  any  marked  alterations  in  the  lumen  of  the  tube 
occurs — either  a  sudden  narrowing  or  a  sudden  expansion — 


234  HUMAN   PHYSIOLOGY 

the  flow  of  fluid  becomes  vibratory,  and,  setting  up  vibrations 
in  the  solid  tissues,  produces  a  musical  sound. 

Such  changes  in  the  calibre  of  the  heart  are  produced  in 
two  ways : — 

1st.  By  a  narrowing,  either  absolute  or  relative,  of  the 
orifices  between  the  cavities — stenosis. 

2nd.  By  a  non-closure  of  the  valves — incompetence. 

Stenosis. — If  one  of  the  auriculo-ventricnlar  orifices  is 
narrowed,  we  then  hear  the  murmur  during  the  period  at 
which  blood  normally  flows  through  this  opening.  A  refer- 
ence to  Fig.  114  at  once  shows  that  this  occurs  during  the 
whole  of  ventricular  diastole,  and  that  the  flow  is  most 
powerful  during  the  first  period  of  ventricular  diastole  and 
during  auricular  systole. 

If  the  aortic  or  pulmonary  valve  is  narrowed  the  murmur 
will  be  heard  (Fig.  114)  during  ventricular  systole. 

The  narrowing  need  not  be  absolute.  A  dilatation  of  the 
artery  will  make  the  orifice  relatively  narrow,  and  will  produce 
the  same  result. 

Incompetence. — If  the  auriculo-ventricular  valves  fail  to 
close  properly,  then  during  ventricular  systole  blood  will  be 
driven  back  into  the  auricles,  and  a  murmur  will  be  heard 
during  this  period. 

If  the  aortic  or  pulmonary  valves  fail  to  close,  the  blood 
will  regurgitate  into  the  ventricles  from  the  arteries  during 
ventricular  diastole,  and  a  murmur  will  be  heard  during  this 
period. 

By  the  position  of  these  murmurs  the  pathological  con- 
dition producing  them  may  be  determined. 

9.  Work  of  the  Heart. — The  heart  in  pumping  blood 
through  it  is  doing  work,  and  the  amount  of  work  may  be 
expressed  in  work  units — e.g.  kilogram  metres.  With  each 
beat  something  under  80  grms.  of  blood  are  thrown  from 
each  ventricle  into  the  aorta  and  pulmonary  artery.  Thus 
the  weight  lifted  may  be  0*08  kilos.  The  output  of  blood 
at  each  beat  of  the  heart  of  the  dog  is  measured  by  Roy's 
cardiometer,  a  rigid  walled  air-tight  case,  which  is  placed 
round  the  heart  and  connected  with  a  piston-recorder,  so 
that  the  decrease  in  the  volume  of  the  enclosed  heart  due 


THE   CIRCULATION 


235 


to  the  blood  leaving  it  may  be  directly  recorded  by  means 
of  a  lever  attached  to  the  piston. 

The  average  resistance  in  the  aorta  may  be  taken  at  about 
1-5  metres  of  blood.     Hence,  with  each  beat,  the  left  ventricle 
may  perform  0-08  x  1'5  =  0-12   Kgms.  of  work.      The  right 
ventricle  is  only  one-third  as 
strong  as  the  left,  and  hence 
the  work  done  by  each  beat 
is  only  0*04  Kgms. 

If  the  heart  is  beating  72 
times  per  minute,  the  amount 
of  work  per  minute  will  be 
something  under  11 '5  Kgms., 
or  16,560  Kgms.  in  24  hours. 
Some  investigators  estimate 
it  as  low  as  10,000  Kgms. 

In  cardiac  muscle  the 
greater  the  resistance  to  con- 
traction the  stronger  the  force 
of  contraction.  Hence,  when 
extra  blood  is  poured  into  the 
heart  from  the  veins,  or  when 
the  outflow  from  the  ventricles 
into  the  arteries  is  impeded, 
the  increased  strain  put  upon 
the  heart  muscle  is  met  by 
increased  contraction,  and  the 
additional  work  thrown  upon 

the  organ  is  effectually  performed.  Not  only  is  this  the 
case  when  temporary  disturbances  of  the  circulation  occur, 
but  when  these  disturbances  are  permanent,  the  heart  adapts 
itself  to  them,  and,  if  it  has  continuously  to  perform  extra 
work,  its  muscular  wall  hypertrophies,  just  as  the  skeletal 
muscles  grow  by  continual  use.  Of  course,  to  allow  such 
compensation  to  be  established,  the  blood  supply  to  the 
heart  muscle  must  be  sufficient,  and  hence,  when  the 
coronary  arteries  are  diseased,  heart  failure  rapidly  ensues. 
If  the  coronary  arteries  are  clamped  and  then  relaxed, 
a  peculiar  fibrillar  contraction  of  the  heart  muscle 
occurs. 


FIG.  116.  —  Eoy's  Cardiometer  to 
measure  the  output  of  blood  from 
the  heart.  b,  heart  in  cardio- 
meter  chamber ;  c,  piston  re 
corder  working  on  lever  against 
rubber  band,  d. 


236  HUMAN   PHYSIOLOGY 

10.  Nature  of  Cardiac  Contraction. —The  contraction  of  the 
ventricle  lasts  for  a  considerable  period — 0*3  seconds.     Is  it 
of  the  nature  of  a  single  contraction,  or  of  a  tetanus  ? 

It  is  impossible  to  tetanise  heart  muscle,  even  by  rapidly 
repeated  induction  shocks.  A  single  stimulus  applied  to  heart 
muscle  produces  a  single  prolonged  contraction.  Again,  the 
mode  of  development  of  the  currents  of  action  does  not  indi- 
cate anything  of  the  nature  of  a  tetanus.  With  each  beat  of 
the  ventricles  the  variation  in  the  electric  potential  begins 
at  the  base  and  travels  rapidly  to  the  apex.  This  passage 
of  the  contraction  wave  along  the  fibres  explains  the  great 
length  of  the  ventricular  systole  as  a  whole.  There  can  be 
no  doubt  that  each  contraction  of  heart  muscle  is  of  the 
nature  of  a  muscle  twitch.  In  this  respect  heart  muscle 
resembles  non-striped  muscle. 

It  further  resembles  it  in  that  the  minimum  stimulus  is 
also  a  maximum  stimulus — i.e.  the  smallest  stimulus  which 
will  make  the  muscle  contract  makes  it  contract  to  the 
utmost.  But  while  this  is  the  case  the  strength  of  stimulus 
necessary  to  call  forth  a  contraction  varies  at  different 
periods.  To  produce  another  contraction  while  the  muscle  is 
alreadyjin  the  period  of  contraction  is  difficult,  but  as  it  relaxes 
it  reacts  more  and  more  readily  to  stimuli  In  cardiac  muscle, 
perhaps  more  than  in  any  other,  the  staircase  increase  in  the 
extent  of  contraction  with  a  series  of  stimuli  is  manifested. 

11.  How   is   the    Rhythmic    Contraction    of    the    Heart 
maintained? — The    mechanism   is   in   the  heart  itself,  for 
the  excised  heart  continues  to  beat. 

In  considering  what  this  mechanism  is,  it  must  be  borne 
in  mind^that  two  distinct  questions  have  to  be  investigated. 

1st.  How  does  the  contraction,  once  started,  pass  in  regular 
sequence  from  one  part  of  the  heart  to  the  other  ? 
2nd.  What  starts  each  rhythmic  contraction  ? 

1st.  Propagation  of  the   Wave  of  Contraction. — In  the 

heart  of  many  of  the  lower  animals,  and  in  the  embryo  of 
mammals,  no  nervous  structures  are  to  be  found,  and  the 
rhythmic  contraction  is  manifestly  simply  a  function  of  the 
muscular  fibres. 


THE  CIRCULATION  237 

Even  in  the  heart  of  animals  with  well-marked  nerve  cells 
in  the  walls  of  the  heart,  and  with  nerve  fibres  coursing 
among  the  muscular  fibres,  the  conduction  of  the  contraction 
is  purely  a  function  of  the  muscles.  For  if  the  heart  of  a 
frog  be  cut  across  and  across,  so  that  all  nerve  fibres  are 
severed,  the  contraction  passes  along  it.  The  rate  at  which 
the  contraction  travels  is  slow,  only  about  10  to  15  centi- 
metres per  second. 

Since  in  the  mammalian  heart  muscular  continuity  be- 
tween auricles  and  ventricles  is  partially  interrupted,  the 
wave  of  contraction  is  delayed  at  this  point,  and  in  the  dying 
heart,  and  in  various  pathological  conditions,  the  contraction 
frequently  fails  altogether  to  pass  this  block,  and  thus  the 
ventricles  either  stop  beating  before  the  auricles,  or  respond 
to  every  second  or  third  auricular  contraction. 

2nd.  Starting  Mechanism  of  Contraction. — In  the  ascidian 
heart  no  nerve  structures  have  been  found,  yet  it  beats 
regularly  and  rhythmically.  In  the  apex  of  the  ventricle 
of  the  frog  there  are  no  nerve  structures,  yet,  if  the  apex  be 
cut  off  and  repeatedly  stimulated  at  regular  intervals  with 
galvanic  making  and  breaking  stimuli,  it  will,  after  a  time, 
begin  to  contract  spontaneously,  regularly,  and  rhythmically. 
Not  only  so,  but  if  the  apex  be  tied  on  to  a  tube,  and  a 
stream  of  blood  passed  through  it,  it  will  again  start  con- 
tracting regularly  and  rhythmically. 

These  experiments  clearly  show  that  regular  rhythmic 
contraction  is  a  function  of  cardiac  muscle. 

In  the  cardiac  cycle  in  the  frog  each  contraction  starts  in 
the  sinus.  What  part  does  the  sinus  take  in  initiating 
contraction  ? 

If  a  ligature  be  tightly  applied  between  the  sinus  and 
auricles  (Stannius'  Experiment),  the  sinus  continues  to 
beat,  and  the  auricles  and  ventricles  usually  stop  beating 
for  a  longer  or  shorter  period.  But  ultimately  they  begin 
to  beat  again.  Hence  it  would  seem  that  it  is  not  any 
special  mechanism  in  the  sinus  which  is  essential  in  starting 
cardiac  contraction.  A  ligature  subsequently  applied  between 
auricles  and  ventricles  sometimes  starts  the  one,  sometimes 
the  other,  sometimes  neither.  Hence  we  see  that  any 
part  of  the  heart  has  the  power  of  originating  rhythmical 


HUMAN   PHYSIOLOGY 


contractions,  although  usually  the  sinus  initiates  it.  The 
sinus  more  than  any  other  part  of  the  heart  has  the 
property  of  rhythmic  contraction  (Practical  Physiology, 
Chap.  IX.). 

We  have  no  evidence  that  the  nerve  cells  in  the  sinus  or 
elsewhere  have  anything  to  do  with  this ;  and  so  far  as  we 
at  present  know,  the  initiation  as  well  as  the  propagation  of 
the  cardiac  contraction  is  a  function  of  the  muscular  fibres. 

3rd.  Intra-cardiac  Nervous  Mechanism. — In  the  frog's 
heart  nervous  structures  exist,  and  are  distributed  as 
follows : — 

1st.  In  the  wall  of  the  sinus  venosus  there  are  a  number  of 


AOFTTA 


V.C.I. 


FlQ.  117. — Scheme  of  the  various  chambers  of  the  Frog's  Heart  and  of  the 
distribution  of  the  intracardiac  nervous  mechanism. 

nerve  cells  constituting  the  ganglion  of  the  sinus  (Remak's 
ganglion). 

2nd.  In  the  inter-auricular  septum  a  number  of  nerve  cells 
constitute  the  ganglion  of  the  auricular  septum. 

3rd.  In  the  auriculo-ventricular  groove  a  number  of 
nerve  cells  are  also  found  forming  the  auriculo-ventricular 
ganglion  (Bidder's  ganglion).  With  these  intra-cardiac 
ganglia  the  terminations  of  the  vagi  nerves  form  definite 
synapses. 

In  the  mammalian  heart  nerve  cells  exist,  but  there  is  not 


THE   CIRCULATION  239 

the  same  differentiation  into  distinct  groups.  Nevertheless 
they  are  abundant  round  the  mouths  of  the  great  veins, 
round  the  edges  of  the  inter- auricular  septum,  and  round 
the  auriculo-ventricular  groove. 

While  there  is  no  evidence  that  the  nervous  structures 
play  an  important  part  in  starting  or  keeping  up  the  con- 
tractions, there  is  evidence  that  they  exercise  a  checking  or 
controlling  action. 

If  the  region  between  the  sinus  and  auricles  in  the  frog's 
heart  is  stimulated  by  the  interrupted  current  from  an 
induction  coil,  the  heart  is  slowed  or  stopped.  (Practical 
Physiology,  Chap.  IX.) 

If  atropine  is  first  applied  electric  stimulation  is  without 
result.  (Practical  Physiology,  Chap.  IX.) 

These  experiments  seem  to  indicate  that  there  is  in  the 
heart  a  checking  mechanism  which  may  be  stimulated  by 
electricity,  and  which  is  paralysed  by  atropine. 

4:th.  Connections  of  the  Heart  with  the  Central  Nervous 
System. — In  the  frog  a  branch  from  the  vagus  connects  the 
central  nervous  system  with  the  heart.  When  the  branch  is 
cut  no  effect  is  produced,  showing  that  it  is  not  constantly 
in  action ;  but  when  the  lower  end  is  stimulated,  the  heart  is 
generally  slowed  or  brought  to  a  standstill.  Sometimes  the 
effect  is  not  marked.  The  reason  for  this  is  that  the  cardiac 
branch  of  the  vagus  in  the  frog  is  really  a  double  nerve 
derived  in  part  from  the  spinal  accessory  and  in  part  from 
fibres  Avhich  reach  the  vagus  from  the  superior  thoracic  sym- 
pathetic ganglion.  If  the  spinal  accessory  is  stimulated,  the 
heart  is  always  slowed ;  and  if  the  sympathetic  fibres  are 
stimulated,  it  is  quickened.  Generally  stimulation  of  the 
cardiac  branch  containing  these  two  sets  of  fibres  simply 
gives  the  result  of  stimulating  the  former,  but  sometimes 
the  stimulation  of  the  latter  masks  this  effect.  (Practical 
Physiology,  Chap.  IX.) 

In  the  mammal  three  sets  of  nerve  fibres  pass  to  the 
heart : — 

1st.  Superior  cardiac  branch  of  the  vagus  starts  from  near 
the  origin  of  the  superior  laryngeal  nerve,  and  passes  to  the 
heart  to  end  in  the  endocardium  (Fig.  118,  S.C.). 


240 


HUMAN   PHYSIOLOGY 


s.c 


2nd.  Inferior  cardiac  branch  of  the  vagus  leaves  the  main 

nerve  near  the  re- 
current laryngeal, 
and  passes  to  join 
the  superficial  car- 
diac plexus  in  the 
heart  (Fig.  118,/.(7.). 
3rd.  Sympathetic 
nerve  fibres  come 
from  the  superior 
thoracic  and  in- 
ferior cervical  gan- 
glia, and  also  end 
in  the  superficial 
cardiac  plexus  (Fig. 
118,  8.). 

Functions  of  the 
Cardiac  Nerves. — 
A.  The  Superior 
Cardiac  Branch  of 
the  Yagus  is  an  in- 
going nerve.  Sec- 
tion produces  no 
effect ;  stimulation 
of  the  lower  end 
causes  no  effect ; 
stimulation  of  the 
upper  end  causes 
slowing  of  the  heart 
and  a  marked  fall 
in  the  pressure 
of  blood  in  the 
arteries,  and  it 
may  cause  pain. 


Flo.  118.— Connections  of  the  Heart  with  the  Central 
Nervous  System.  Au.,  Auricle;  V.,  Ventricle; 
V.D.C.y  Abdominal  Vaso-dilator  Centre;  C.I.C., 
Cardiac  Inhibitory  Centre;  C.A.C.,  Cardio-aug- 
mentor  Centre;  S.C.,  Superior  Cardiac  Branch  of 
the  Vagus;  7.C7.,  Inferior  Cardiac  Branch  of  the 


Vagus  with  Cell  Station  in  the  Heart;  S.,  Cardio-  The  slowing  of  the 
sympathetic  Fibres  with  Cell  Station  in  the  Len-  heart  is  a  reflex 
ticular  Ganglion;  V. D. A b. ,  Vaso-dilator  Fibres 
to  Abdominal  Vessels.  The  continuous  lines  are 
outgoing ;  the  broken  lines  are  ingoing  Nerves. 


effect   through  the 
inferior    cardiac 
branch ;     and     the 
fall  of  blood  pressure,  which  is  the  most  manifest  effect,  is 


THE   <2!ROTLATMT  .,  241 

\   O  V 


due  to  a  reflex  dilatation  of  the  vessels  of  the  abdomen, 
causing  the  blood  to  accumulate  there,  and  thus  to  lessen 
the  pressure  in  the  arteries  generally.  On  account  of  the 
action  in  the  blood  pressure,  it  is  called  the  depressor 
nerve. 

B.  Inferior  Cardiac  Branch  of  Yagus.  —  Section  of  the 
vagus  or  of  this  branch  causes  acceleration  of  the  action 
of  the  heart.  The  nerve  is  therefore  constantly  in  action. 
Stimulation  of  its  central  end  has  no  effect;  stimulation 
of  its  peripheral  end  causes  a  slowing  or  stoppage  of  the 
heart.  It  is  therefore  the  checking  or  inhibitory  nerve  of 
the  heart. 

1.  dwtse.tf  the  Fibres.  —  These  fibres  leave  the  central  ner- 
vous system  by  the  spinal  accessory,  and  pass  to  the  heart 
to  form  synapses  with  the  cells  of  the  cardiac  plexuses. 

2.  Centre.  —  The  fibres  arise  from  a  centre  in  the  medulla 
oblongata,  which  can   be  stimulated  to   increased   activity 
either  directly  or  reflexly.     (1)  Direct  stimulation  is  brought 
about  by  (a)  sudden  anaemia  of  the  brain,  as  when  the  arteries 
to  the  head  are  clamped  or  occluded  ;  (b)  increased  venosity 
of  the  blood,  as  when  respiration  is  interfered  with  ;  (c)  the 
concurrent  action  of  the  respiratory  centre  (see  p.  295).     (2) 
Reflex  stimulation  is  produced  through  many  nerves.     In 
the  rabbit  stimulation  of  the  5th  cranial  nerve  by  the  inhala- 
tion of  ammonia  vapours  has  this  action,  and  in  all  animals 
stimulation  of  the  abdominal  nerves  produces  the  same  effect. 
This  reflex  stimulation  of  the  centre  is  used  to  determine  its 
position  in  the  medulla.     It  can  be  induced  after  removal  of 
the  brain  above  the  medulla,  but  destruction  of  the,  medulla 
entirely  prevents  it. 

3.  Mode  of  Action.  —  These  inhibitory  fibres  appear  to  act 
by  stimulating  the  local  inhibitory  mechanism  in  the  heart  ; 
and  when  this  has  been  poisoned  by  atropine,  they  cannot 
act.     According  to  the  observation  of  Gaskell,  they  excite 
in  the  heart  anabolic  changes,  since  the  electric  current  of 
injury  is  increased  when  they  are  stimulated,  indicating  that 
the  difference  between  the  living  part  of  the  heart  and  the 
injured  part  is  increased  (see  p.  65). 

4.  Result  of  Action. 

(a)  The  output  of  blood-  from  the  heart  is  diminished,  and 

16 


242 


HUMAN   PHYSIOLOGY 


thus  less  blood  is  forced  into  the  arteries,  and  the  blood 
pressure  falls. 

(6)  The  rhythm  of  both  auricles  and  ventricles  is  slowed, 
but  the  effect  on  the  auricles  is  more  marked  than  upon  the 
ventricles,  and  the  ventricles  may  show  a  contraction  rhythm 
independent  of  that  of  the  auricles  (Fig.  119,  A.). 

(c)  The  force  of  contraction  of  the  auricles  is  decreased. 


FIG.  119. — Simultaneous  Tracing  from  Auricles  and  Ventricles.  A.,  During  Stimu- 
lation of  the  Vagus;  B.t  During  Stimulation  of  the  Sympathetic.  Each 
downstroke  marks  a  systole,  each  upstroke  a  diastole.  (From  ROY  and 
ADAHI.  ) 


In  the  ventricles  the  systole  becomes  less  complete  and  the 
cavities  become  more  and  more  distended.  In  the  heart 
of  the  tortoise  excitability  and  conductivity  are  decreased, 
and  the  auricular  contraction  may  fail  to  pass  to  the  ven- 
tricles. 


THE   CIRCULATION  243 

C.  Sympathetic  Fibres. — The  outgoing  fibres  are  the 
augmentors  and  accelerators  of  the  heart's  action.  When 
they  are  cut,  no  effect  is  produced,  therefore  the  centre 
is  not  constantly  in  action;  but  when  the  peripheral 
end  is  stimulated,  the  rate  and  force  of  the  heart  are 
increased. 

1.  Course   of   the    Fibres. — These   are   small  medullated 
fibres.    They  leave  the  spinal  cord  by  the  anterior  roots  of  the 
2nd,  3rd,  and  4th  dorsal  nerves  passing  to  the  stellate  gang- 
lion where  they  have  their  cell  stations  (Fig.  118).    From  the 
cells  in  this  ganglion  non-medullated  fibres  run  on  in  the 
annulus  of  Vieussens,  and  from  this  and  from  the  inferior 
cervical  ganglion  they  pass  out  to  the  muscular  fibres   of 
the  heart. 

2.  The  Centre  is  in  the  medulla,  and  it  may  be  stimulated 
by  stimulating  various  ingoing  nerves,  such  as  the  sciatic ;  or 
it  may  be  set  in  action  from  the  higher  nerve  centres  in 
various  emotional  conditions. 

3.  Mode   of  Action. — These   fibres   seern   to   act   directly 
upon  the  muscular  fibres,  increasing  their  excitability  and 
conductivity. 

4.  Result  of  Action — 

(a)  The  output  of  blood  from  the  heart  is  increased,  and 
the  pressure  of  £>lood  in  the  arteries  is  raised. 

(6)  The  rate  of  the  rhythmic  movements  of  auricles  and 
ventricles  is  increased. 

(c)  The  force  of  contraction  of  auricles  and  ventricles  is 
increased. 

It  is  probable  that  the  cardiac  sympathetic  also  carries 
ingoing  fibres  which  enter  the  cord  in  the  lower  cervical 
region.  The  pain  experienced  in  the  arm  in  heart  disease 
is  generally  thought  to  be  due  to  the  implication  of  these 
fibres  leading  to  the  sensation  which  is  referred  to  the  cor- 
responding somatic  nerves  (p.  146). 

The  vagus  is  thus  the  protecting  nerve  of  the  heart, 
reducing  its  work  and  diminishing  the  pressure  in  the 
arteries. 

The  sympathetic  is  the  whip  which  forces  the  heart  to 
increased  action  in  order  to  keep  up  the  pressure  in  the 
arteries. 


244  HUMA      PHYSIOLOGY 


III.  Circulation  in  the  Blood  Vessels. 

The  general  distribution  of  the  various  vessels — arteries, 
capillaries,  veins,  and  lymphatics — has  been  already  con- 
sidered (Fig.  105,  p.  209). 

(The  structure  of  the  walls  of  each  must  be  studied 
practically.) 

The  capillaries  are  minute  tubes  of  about  12  micro- 
millimetres  in  diameter,  forming  an  anastomising  network 
throughout  the  tissues.  Their  wall  consists  of  a  single 
layer  of  endothelium.  On  passing  from  the  capillaries  to 
arteries  on  the  one  side,  and  to  veins  and  lymphatics  on 
the  other,  non-striped  muscle  fibres  make  their  appearance 
encircling  the  tube.  Between  these  fibres  and  the  endo- 
thelium a  fine  elastic  membrane  next  appears,  while  out- 
side the  muscles  a  sheath  of  fibrous  tissue  develops.  Thus 
the  three  essential  layers  of  the  coats  of  these  vessels  are 
produced  :— 

Tunica  intima,  consisting  of  endothelium  set  on  the  in- 
ternal elastic  membrane. 

Tunica  media,  consisting  chiefly  of  the  visceral  muscular 
fibres. 

Tunica  adventitia,  consisting  of  loose  fibrous  tissue. 

The  coats  of  the  arteries  are  thick ;  those  of  the  veins  are 
thin.  In  the  large  arteries  the  muscular  fibres  of  the  media 
are  largely  replaced  by  elastic  fibres  so  that  the  vessels  may 
better  stand  the  strain  of  the  charge  of  blood  which  is  shot 
from  the  heart  at  each  contraction.  In  the  veins  double 
flaps  of  the  tunica  intima  form  valves  which  prevent  any 
regurgitation  of  blood. 

The  great  characteristic  of  the  walls  of  the  large  arteries 
is  the  toughness  and  elasticity  given  by  the  abundance  of 
elastic  fibrous  tissue,  of  the  small  arteries,  the  contractility 
due  to  the  preponderance  of  muscular  fibres. 

The  circulation  of  blood  in  the  vessels  is  that  of  a  fluid  in 
a  closed  system  of  elastic-walled  tubes,  at  one  end  of  which 
(the  great  arteries)  a  high  pressure,  and  at  the  other  (the 
great  veins)  a  low  pressure  is  kept  up.  As  a  result  of  this 


m 

distribution  of  pressure  there  is  a  constant  flow  of  blood 
from  arteries  to  veins. 

Many  points  in  connection  with  the  circulation  may  be 
conveniently  studied  on  a  model  made  of  indiarubber  tubes 
and  a  Higginson's  syringe.  (Practical  Exercise.) 

A.  —  Blood  Pressure. 

The  distribution  of  pressure  is  the  cause  of  the  flow  of 
blood,  and  must  first  be  considered. 


1.   General  Distribution  of  Pressure. 

(See  Fig.  125,  p.  258.) 

The  pressure  in  any  part  of  a  system  of  tubes  depends 
upon  two  factors  — 

1st.  The  force  propelling  fluid  into  that  part  of  the 
tubes. 

2nd.  The  resistance  to  the  outflow  of  fluid  from  that  part 
of  the  tubes. 

The  pressure  in  the  arteries  is  high,  because  with  each 
beat  of  the  heart  about  80  grms.  of  blood  are  thrown  with 
the  whole  contractile  force  of  each  ventricle  into  the  cor- 
responding artery;  and  because  the  resistance  offered  to 
the  outflow  of  blood  from  the  arteries  into  the  capillaries 
and  veins  is  enormous.  For,  as  the  blood  passes  into 
innumerable  small  vessels,  it  is  subjected  to  greater  and 
greater  friction  —  just  as  a  river  in  flowing  from  a  deep 
narrow  channel  on  to  a  broad  shallow  bed  is  subjected  to 
greater  friction. 

Thus  in  the  arteries  the  powerful  propulsive  force  of  the 
heart  and  the  great  resistance  to  outflow  keep  the  pressure 
high. 

When  the  capillaries  are  reached  much  of  the  force  of 
the  heart  has  been  lost  in  dilating  the  elastic  coats  of  the 
arteries,  and  thus  the  inflow  into  the  capillaries  is  much 
weaker  than  the  inflow  into  the  arteries.  At  the  same 
time  the  resistance  to  outflow  is  small,  for  in  passing  from 
capillaries  to  veins  the  channel  of  the  blood  is  becoming 


246  HUMAN   PHYSIOLOGY 

less  broken  up  and  thus  opposes  less  friction  to  the  inflow 
of  the  blood. 

When  the  veins  are  reached  the  propelling  force  of  the 
heart  is  still  further  weakened,  and  hence  the  force  of  inflow 
is  very  small.  But,  instead  of  there  being  a  resistance  to 
outflow  from  the  veins  into  the  heart,  this  is  favoured  by 
the  suction  action  of  the  heart  during  diastole,  and  also  by 
the  fact  that  the  great  veins,  in  entering  the  heart,  pass  into 
the  thorax,  an  air-tight  box  in  which  during  each  inspiration 
a  very  low  pressure  is  developed. 

What  has  been  said  of  the  veins  applies  equally  to  the 
lymphatics. 

2.  Variations  in  Blood  Pressure. 

Before  considering  the  exact  measurements  of  pressure  in 
these  different  vessels,  the  rhythmic  variations  in  pressure 
may  be  considered. 

I.  Synchronous  with  the  Heart  Beats. 

A.  Arterial  Pulse. 

If  the  finger  be  placed  on  any  artery,  a  distinct  expansion 
will  be  felt  following  each  ventricular  systole. 

This  expansion  develops  suddenly  and  disappears  more 
slowly.  In  some  cases  it  may  be  felt  by  simply  laying  the 
finger  on  the  surface  of  the  artery  without  exerting  pressure, 
in  other  cases  it  may  be  necessary  to  compress  the  artery 
before  the  pulsation  is  distinctly  felt. 

If  any  vein  be  investigated  in  the  same  way  it  will  be 
found  that  no  pulse  can  be  detected.  In  the  capillaries  too 
this  pulse  does  not  exist. 

It  is  best  marked  in  the  great  arteries,  and  becomes  less 
and  less  distinct  as  the  small  terminal  arteries  are  reached. 

Cause  of  Pulse. — The  arterial  pulse  is  due  to — 

1st.  The  intermittent  inflow  of  blood.  The  arteries  expand 
with  each  sudden  inflow  of  80  grms.  of  blood  from  the  heart 
into  the  arterial  system. 

2nd.  The  resistance  to  outflow  from  the  arteries  into  the 
capillaries. 

If  blood  could  flow  freely  from  the  arteries  into  the  capil- 


THE  CIRCULATION  247 

laries,  then  the  inrush  of  blood  from  the  heart  would  simply 
displace  the  same  amount  of  blood  into  the  capillaries  and 
the  arteries  would  not  be  expanded.  As  already  indicated, 
the  friction  between  the  walls  of  the  innumerable  small 
arterioles  and  the  blood  is  so  great  that  the  flow  out  of  the 
arteries  is  not  so  free  as  to  allow  the  blood  to  pass  into  the 
capillaries  so  rapidly  as  it  is  shot  into  the  arteries.  Hence 
with  each  beat  of  the  heart  an  excess  of  blood  must 
accumulate  in  the  arteries. 

3rd.  To  allow  of  their  expanding  to  accommodate  this 
excess  of  blood  their  walls  must  be  elastic. 

It  is  upon  these  three  factors  that  the  arterial  pulse  depends. 
Do  away  with  either,  and  the  pulse  at  once  disappears. 

Why  is  there  no  Pulse  in  the  Veins? — Their  walls  are 
elastic,  but,  in  the  first  place,  instead  of  there  being  an 
obstruction  to  the  outflow  of  blood  from  the  veins  into  the 
heart,  this  is  favoured  by  the  suction  action  of  the  heart  and 
thorax.  Hence,  even  if  an  intermittent  inflow  were  well 
marked,  the  absence  of  resistance  to  outflow  would  in  itself 
prevent  the  development  of  a  venous  pulse.  But  the  inflow 
is  not  intermittent.  With  each  beat  of  the  heart  the  blood 
does  not  pass  freely  from  the  arteries  into  the  capillaries  and 
veins,  but  it  only  slowly  escapes,  just  as  much  passing  out 
between  the  beats  as  during  the  beats.  Hence  the  most 
important  factor  in  causing  a  pulse,  an  intermittent  inflow, 
is  absent. 

With  no  sudden  intermittent  inflow,  and  with  no  resist- 
ance to  outflow,  the  development  of  a  pulse  is  impossible. 

In  certain  abnormal  conditions,  where,  from  the  extreme 
dilatation  of  the  arterioles,  the  inflow  into  the  veins  is  very 
free,  and  where  the  outflow  from  the  part  of  the  body  is  not 
so  free,  a  local  venous  pulse  may  develop. 

Characters  of  the  Pulse  Wave. — If  a  finger  be  placed  on 
the  carotid  artery  and  another  upon  the  radial  artery  it  will 
be  felt  that  the  artery  near  the  heart  expands  (pulses)  before 
that  further  from  the  heart. 

The  pulse  develops  first  in  the  arteries  near  the  heart 
and  passes  outwards  towards  the  periphery.  The  reason  for 
this  is  obvious.  The  arteries  are  always  overfilled  with  blood. 
The  ventricle  drives  its  contents  into  this  overfilled  aorta, 


248  HUMAN   PHYSIOLOGY 

and  to  accommodate  this  the  aortic  wall  expands.  But  since 
the  aorta  communicates  with  the  other  arteries  this  increased 
pressure  passes  outwards  along  them  expanding  their  wall  as 
it  goes. 

The  pulse  wave  may  thus  be  compared  to  a  wave  at  sea, 
which  is  also  a  wave  of  increased,  pressure,  the  only  differ- 
ence being  that,  while  the  waves  at  sea  travel  freely  over  the 
surface,  the  pulse  wave  is  confined  in  the  column  of  blood, 
and  manifests  itself  by  expanding  the  walls  of  the  arteries. 

It  greatly  simplifies  the  study  of  the  pulse  to  regard  it  in 
this  light,  and  to  study  it  just  as  we  would  study  a  wave 
at  sea. 

1.  Velocity. — To  determine  how  fast  a  wave  is  travelling 
we  might  select  two  points  at  a  known  distance  from  one 
another,  and  with  a  watch  note  how  long  the  wave  took  to 
pass  from  one  to  the  other.     So  with  the  pulse  wave,  two 
points  on  an  artery  at  a  known  distance  from  one  another 
may  be  taken  and  the  time  which  the  wave  takes  to  pass 
between  them  may  be  measured. 

It  is  thus  found  that  the  pulse  wave  travels  at  about  9  or 
10  metres  per  second — about  thirty  times  as  fast  as  the  blood 
flows  in  the  arteries. 

2.  Length  of  the  Wave. — To  determine  this  in  a  wave  at 
sea  is  easy  if  we  know  its  velocity  and  know  how  long  it 
takes  to  pass  any  one  point.      Suppose  it  is  travelling  at 
50  feet  per  second,  and  that  it  takes  1  second  to  pass  a  par- 
ticular point,  obviously  it  is  50  feet  in  length.     The  same 
method  may  be  applied  to  the  pulse  wave.     We  know  its 
velocity,  and  by  placing  the  finger  on  an  artery  we  may 
determine  that  one  wave  follows  another  in  rapid  succession, 
so  that  there  is  no  pause  between  them.     Each  wave  corre- 
sponds to  a  ventricular  systole,  and  therefore  each  wave  must 
last,  at  any  point,  just  the  time  between  two   ventricular 
systoles — just  the  time  of  a  cardiac  cycle.     There  are  about 
70  cycles  per  minute — i.e.  per  60  seconds  ;  hence  each  must 
last  0-88  second.     The  pulse  wave  takes  0'8S  second  to  pass 
any  place,  and  it  travels  at  30  feet  per  second ;  its  length  then 
is  26*4  feet,  or  about  Jive  times  the  length  of  the  body.     It  is 
then  an  enormously  long  wave,  and  it  has  disappeared  at 
the  periphery  long  before  it  has  finished  leaving  the  aorta. 


THE   CIRCULATION  249 

3.  The  Height  of  the  Wave. — The  height  of  the  pulse  wave, 
as  of  the  wave  at  sea,  depends  primarily  on  the  pressure  caus- 
ing it,  but  the  character  of  the  arterial  wall  modifies  it  very 
largely.     Thus  the  true  height  of  the  pulse  wave  in  the  great 
arteries  near  the  heart  is  masked  by  the  thickness  of  the 
arterial  wall. 

Speaking  generally,  however,  we  may  say  that  the  pulse 
wave  is  highest  near  the  heart,  and  becomes  lower  and  lower 
as  we  pass  out  to  the  periphery,  where,  as  already  seen,  it 
finally  disappears  altogether.  This  disappearance  is  due  to 
its  force  becoming  expended  in  expanding  the  arterial  wall. 

4.  The  Form  of  the  Wave. — Waves  at  sea  vary  greatly  in 
form,  and  the  form  of  the  wave  might  be  graphically  recorded 
on  some  moving  surface  like  the  side  of  a  ship  by  some  float- 
ing body.     If  the  ship  were  stationary  a  simple  vertical  line 
would  be  produced,  but  if  she  were  moving  a  curve  would  be 
recorded,  more  or  less  abrupt  according  to  her  speed.     From 
this  curve  the  shape  of  the  wave  might  be  deduced,  if  we 
knew  the  speed  of  the  vessel. 

The  same  method  may  be  applied  to  the  arterial  pulse. 
By  recording  the  changes  produced  by  the  pulse  wave  as  it 
passes  any  point  in  an  artery  the  shape  of  the  wave  may 
be  deduced. 

This  may  be  done  by  any  of  the  various  forms  of  sphygmo- 
graph.  (Practical  Physiology,  Chap.  XI.) 

Such  a  tracing  is  not  a  true  picture  of  the  wave,  but 
simply  of  the  effect  of  the  wave  on  one  point  of  the  arterial 
wall.  Its  apparent  length  depends  upon  the  rate  at  which 
the  recording  surface  is  travelling  and  not  on  the  length  of 
the  wave. 

Its  height  depends  in  part  upon  the  length  of  the  recording 
lever,  in  part  upon  the  resistance  offered  by  the  instrument, 
in  part  upon  the  degree  of  pressure  with  which  the  instru- 
ment is  applied  to  the  artery,  and  in  part  on  the  thickness  of 
the  arterial  wall. 

Such  a  trace  shows  (Fig.  120) — 

1st.  That  the  pulse  waves  follow  one  another  without  any 
interval. 

2nd.  That  the  rise  of  the  wave  is  much  more  abrupt  than 
the  fall. 


250  HUMAN   PHYSIOLOGY 

3rd.  That  upon  the  descent  of  the  primary  wave  there  are 
one  or  more  secondary  waves. 

One  of  these  is  constant  and  is  often  very  well  marked. 
It  forms  a  second  crest,  and  is  hence  called  the  dicrotic  wave. 

Between  the  chief  crest  and  this  secondary  crest,  a  smaller 
crest  is  often  manifest  (Fig.  120, 3),  and,  from  its  position,  it  is 
called  the  predicrotic  wave.  Sometimes  other  crests  appear. 


Medium   Pressure. 


Medium  Pressure. 


Low  Pressure. 

FlG.  120. — Three  Sphygmographic  Tracings  made  from  the  radial  artery  of  a 
healthy  man  in  the  course  of  one  hour  without  removing  the  Sphymograph. 
1  was  made  immediately  after  muscular  exercise  ;  2  was  made  after  sitting 
still  for  half-an-hour ;  and  3,  after  an  hour. 

If  the  wave  has  only  one  crest  it  is  called  a  one-crested  or 
monocrotic  wave.  If  only  the  dicrotic  crest  is  well  marked 
it  is  called  dicrotic.  If  three  crests  are  present,  tricrotic ; 
if  several  crests,  polycrotic. 

To  understand  the  various  parts  of  the  pulse  wave  it  is 
necessary  to  compare  it  with  the  intra-ventricular  pres- 
sure changes.  This  may  be  done  by  taking  synchronously 
tracings  of  the  intra-ventricular  pressure,  and  of  the  aortic 
pressure  (Fig.  121). 


THE   CIRCULATION  251 

Such  a  tracing  shows  that  at  the  moment  of  ventricular 
systole  the  pressure  in  the  aorta  is  higher  than  that  in  the 
left  ventricle. 

As  ventricular  systole  advances  the  intra-ventricular  pres- 
sure rises  and  becomes  higher  than  the  aortic.  At  that 
moment  the  aortic  valves  are  thrown  open  and  a  rush  of 
blood  takes  place  into  the  aorta,  raising  the  pressure  and 
expanding  the  artery,  and  causing  the  upstroke,  and  crest  of 
the  pulse  curve.  As  the  ventricle  empties  itself  the  intra- 
ventricular  pressure  tends  somewhat  to  fall,  and,  at  the  same 
time,  a  fall  in  the  intra-aortic  pressure  also  takes  place.  If 
all  the  blood  does  not  leave  the  ventricle  in  the  first  gush, 
e.g.  when  the  intra-aortic  pressure  is  high  as  compared  with 
the  force  of  the  heart  (Fig.  121,  continuous  line),  there  is  a 
residual  outflow  which  arrests  the  diminution  in  the  aortic 
pressure,  or  may  actually  raise  it,  causing  the  predicrotic 
wave.  As  this  residual  outflow  diminishes,  the  aortic  pres- 
sure again  falls  and  continues  to  fall  until  the  moment  of 
ventricular  diastole.  At  this  instant  the  intra-ventricular 
pressure  suddenly  becomes  less  than  the  intra-aortic  and  the 
semilunar  valves  are  forced  downwards  towards  the  ventricles, 
and  thus  the  capacity  of  the  aorta  is  slightly  increased  and 
the  pressure  falls.  This  fall  in  pressure  is  indicated  by  the 
dicrotic  notch.  But  the  elasticity  of  the  semilunar  valves  at 
once  makes  them  again  spring  up,  thus  increasing  the  pres- 
sure in  the  aorta  and  causing  the  second  crest,  the  dicrotic 
wave.  After  this  the  pressure  in  the  arteries  steadily 
diminishes  till  the  mean  is  reached,  to  be  again  increased 
by  the  next  ventricular  systole. 

The  form  of  the  pulse  wave  varies  according  to  the 
relationship  between  the  arterial  pressure,  and  the  activity 
of  the  heart. 

If  the  heart  is  active  and  strong  in  relation  to  the  arterial 
pressure,  the  main  mass  of  the  blood  is  expelled  in  the  first 
sudden  outflow,  and  the  residual  flow  is  absent  or  slight 
(Fig.  121,  dotted  line).  In  this  case  there  is  a  sudden  and 
marked  rise  of  the  arterial  pressure,  followed  by  a  steady  fall 
till  the  moment  of  ventricular  diastole.  The  rebound  of  the 
semilunar  valves  is  here  marked  and  causes  a  very  prominent 
dicrotic  wave,  while  the  predicrotic  wave  is  absent  (Fig.  120, 1). 


252 


HUMAN   PHYSIOLOGY 


Such  a  condition  is  well  seen  after  violent  muscular  exertion, 
and  in  certain  fevers.  In  these  conditions  the  dicrotic  wave 
is  so  well  marked  that  it  can  be  readily  perceived  with  the 
finger.  It  is  to  this  form  of  pulse  that  the  term  dicrotic  is 
applied  in  medicine. 

On  the  other  hand,  if  the  ventricles  are  acting  slowly  and 
feebly  in  relationship  to  the  arterial  pressure,  the  initial 
outflow  of  blood  does  not  take  place  so  rapidly  and  com- 
pletely (Fig.  121, 
continuous  line), 
and  the  initial 
rise  in  the  pulse 
is  thus  not  so 
rapid.  The  re- 
sidual outflow  of 
blood  is  more 
marked  and 
causes  a  well- 
marked  secon- 
A5  vs.  V-<>-  ~'~p  dary  rise  in  the 

FIG.   121.— Diagram  to  show  the  effect  of  altering  the  pulse    Curve — the 

relationship  between  the  activity  of  the  Heart  and  predicrotic    Wave. 

the    arterial    blood    pressure.       —  -  —  -  —  b  is   the  y        pprf    Jn      r»flqfts 

curve  of  intra-ventricular  pressure,  and bl  is 

a  pulse  curve  with  an  active  heart  and  a  relatively  this       may      be 

low  arterial    pressure.     —  a   and   a1   are    the    same  hjcrher     than     the 
with  a  sluggish  heart  and  a  relatively  high  arterial 

pressure.  primary  crest, 

producing  the 

condition  known  as  the  anacrotic  pulse.  The  relatively 
high  intra-arterial  pressure  here  prevents  the  development 
of  a  well-marked  dicrotic  wave. 

In  extreme  cases  of  this  kind,  when  the  arterial  walls  are 
very  tense,  they  may  recover  after  their  expansion  in  an 
irregular  jerky  manner,  and  may  give  rise  to  a  series  of  kata- 
crotic  crests  producing  a  polycrotic  pulse  (Fig.  120,  3). 

From  what  has  been  said  it  will  be  seen  that  a  study  of 
the  pulse  wave  gives  most  valuable  information  as  regards 
the  state  of  the  circulation,  and  the  physician  constantly 
makes  use  of  the  pulse  in  diagnosis. 

Palpation  of  Pulse. — On  placing  the  finger  on  the  radial 
artery  the  points  to  determine  are — 


THE   CIRCULATION  253 

1st.  The  rate  of  the  pulse — i.e.  the  rate  of  the  heart's 
action. 

2nd.  The  rhythm  of  the  pulse — i.e.  of  the  heart's  action  as 
regards — (1)  Strength  of  the  various  beats.  Normally  the 
beats  differ  little  from  one  another  in  force — since  the 
various  heart  beats  have  much  the  same  strength.  Respira- 
tion has  a  slight  effect  which  will  afterwards  be  considered 
(see  p.  295).  In  pathological  conditions  great  differences  in 
the  force  of  succeeding  pulse  waves  occur.  (2)  Time  relation- 
ship of  beats.  Normally  the  beats  follow  one  another  at 
regular  intervals — somewhat  shorter  during  inspiration — 
somewhat  longer  during  expiration.  In  pathological  con- 
ditions great  irregularities  in  this  respect  may  occur. 

3rd.  The  volume  of  the  pulse  wave.  Sometimes  the  wave 
is  high  and  greatly  expands  the  artery — sometimes  less  high 
and  expanding  the  artery  less.  The  former  condition  is  called 
a  full  pulse  (pulsus  plenus),  the  latter  a  small  pulse  (pulsus 
parvus).  The  fulness  of  the  pulse  depends  upon  two  factors. 
1st.  The  average  tension  in  the  arteries  between  the  pulse  beats. 
If  this  is  high,  the  walls  of  the  artery  are  already  somewhat 
stretched,  and  therefore  the  pulse  wave  expands  them  further 
only  slightly.  On  the  other  hand,  if  the  average  pressure  is 
low,  the  arterial  wall  is  lax,  and  is  readily  stretched  to  a  greater 
extent.  2nd.  The  force  of  the  heart.  To  stretch  the  arterial 
wall  to  a  large  extent  requires  an  actively  acting  heart  throw- 
ing a  sudden  large  wave  of  blood  into  the  arterial  system  at 
each  systole.  The  full  pulse  is  well  seen  after  violent  exer- 
tion, when  the  heart  is  active  and  the  peripheral  vessels  fully 
dilated,  thus  allowing  a  free  flow  of  blood  from  the  arteries 
and  thus  keeping  the  mean  arterial  pressure  low. 

4:th.  Tension  of  the  pulse.  Sometimes  the  pulse  wave  is 
easily  obliterated  by  pressing  on  the  artery — sometimes  con- 
siderable force  is  required  to  prevent  it  from  passing.  To 
test  this  two  fingers  must  be  placed  on  the  artery.  That 
placed  nearest  the  heart  must  be  pressed  more  and  more 
firmly  on  the  vessel  until  the  pulse  wave  is  no  longer  felt  by 
the  second  finger.  In  this  way  the  tension  or  force  of  the 
pulse  may  be  roughly  determined.  So  important,  however, 
is  this  point  that  various  instrumental  methods  for  deter- 
mining it  have  been  devised. 


254  HUMAN   PHYSIOLOGY 

The  tension  of  the  pulse  varies  directly  with  the  force  of 
the  heart  and  with  the  peripheral  resistance.  The  first 
statement  is  so  obvious  as  to  require  no  amplification.  It  is 
also  clear  that  if  the  peripheral  resistance  is  low  so  that  blood 
can  easily  be  forced  out  of  the  arteries  into  the  capillaries, 
the  arterial  wall  will  not  be  so  forcibly  expanded  as  when 
the  resistance  to  outflow  is  great.  Hence  a  high  tension 
pulse  is  indicative  of  a  strongly  acting  heart  with  constriction 
of  the  peripheral  vessels.  It  is  well  seen  during  the  shivering 
fit  which  so  frequently  precedes  a  febrile  attack,  since  at  that 
time  the  peripheral  vessels  are  constricted  and  the  heart's 
action  excited.  The  tension  of  the  pulse  wave  must  not  be 
confused  with  the  mean  arterial  pressure. 

oth.  The  form  of  the  pulse  wave  may  be  investigated  by 
means  of  the  finger  alone  or  by  means  of  the  sphygmograph. 
The  points  to  be  observed  are — 

(1)  Does  the  wave  come  up  suddenly  under  the  finger  ? 
In  the  pulsus  celer  (or  active  pulse)  it   does  so ;    in   the 
pulsus  tardus,  on  the  other  hand,  it  comes  up  slowly.     The 
former  condition  is  indicative  of  an  actively  acting  heart 
with  no  great  peripheral  resistance — the  latter  indicates  that 
the  heart's  action  is  weak  in  relationship  to  the  arterial  blood 
pressure. 

(2)  Does  the  wave  fall  slowly  or  rapidly  ?     Normally  the 
fall  should   not   be  so   sudden   as  the  ascent.     When  the 
aortic  valves  are  not  closed  properly  the  descent  becomes 
very  rapid. 

(3)  Are  there  any  secondary  waves  to  be  observed  ?     The 
only  one  of  these  which  can  be  detected  by  the  finger  is  the 
dicrotic  wave,  and  this  only  when  it  is  well  marked.     When 
it  can  be  felt,  the  pulse  is  said  to  be  dicrotic,  and,  as  before 
stated,  this  indicates  an  actively  acting  heart  with  an  arterial 
pressure  low  relatively  to  the  strength  of  the  ventricles. 

B.  Capillary  Pulse. 

Normally  there  is  no  pulse  in  the  capillaries.  Their  thin 
endothelial  wall  is  not  well  adapted  to  bear  such  an  inter- 
mittent strain.  If,  however,  the  arterioles  to  a  district  are 
freely  dilated  so  that  little  resistance  is  offered  to  the  escape 


THE  CIRCULATION 


255 


of  blood  from  the  arteries,  and  if  at  the  same  time  the  out- 
flow from  the  capillaries  is  not  proportionately  increased, 
intermittent  inflow  and  resistance  to  outflow  are  developed, 
and  a  pulse  is  produced.  Such  a  condition  is  seen  in  certain 
glands  during  activity. 


C.  Yenous  Pulse. 

1.  The  absence  of  a  general  venous  pulse  has  been  already 
explained.     But  just  as  in  the  capillaries  so  in  the  veins,  a 
local  pulse  may  develop. 

2.  In   the  veins   entering    the   auricles    a    pulse   occurs, 
but  a  pulse  having  no  resemblance  to   the   arterial   pulse, 
although  depend- 
ing on   the   same 

three  factors. 

Its  form  is  indi- 
cated in  Fig.  122. 

Its  features  are 
to  be  explained  as 
follows : — 

Blood  is  con- 
stantly flowing 
into  the  great 
veins,  pressed  on 
by  the  vis  a  tergo. 
When  the  auricles 
contract  the  out- 
flow from  these 
veins  into  the  heart  is  suddenly  checked,  and  consequently 
the  veins  distend.  At  the  moment  of  auricular  diastole 
the  outflow  is  again  free,  a  rush  of  blood  takes  place 
into  the  distending  auricles,  and  thus  the  pressure  in 
the  veins  falls.  But  as  the  ventricular  systole  prevents 
blood  from  passing  through  the  auricles,  a  second  obstruction 
to  outflow  occurs,  and  thus  a  second  increase  in  pressure  is 
developed  in  the  veins.  At  the  moment  when  the  ventricles 
dilate  a  sudden  rush  of  blood  takes  place  from  the  veins  and 
auricles  into  the  ventricles,  and  thus  a  sudden  fall  in  the 
pressure  is  produced.  Gradually,  as  the  ventricles  fill,  the 


FIG.  122. — Tracings  of  the  Pulse  in  the  great  Veins 
in  relationship  to  the  Cardiac  Cycle. 

Normal  Venous  Pulse. 

Venous  Pulse  in  Tricuspid  Incompetence. 


256  PHYSIOLOGY 


pressure  in  the  auricles  and  veins  increases  and  they  are 
again  expanded. 

This  is  the  normal  venous  pulse.  But,  if  the  auriculo-ven- 
tricular  valves  are  incompetent,  when  the  ventricles  contract, 
blood  is  forced  back  into  the  auricles  and  veins  and  a  crest 
develops  between  the  two  normal  crests.  The  height  of  this 
third  crest  is  a  good  index  of  the  amount  of  regurgitation. 


II.  Respiratory  Variations  in  Blood  Pressure. 

Not  only  do  rhythmic  changes  in  the  arterial  pressure  occur 
with  each  beat  of  the  heart,  but  larger  changes  are  caused 
by  the  respirations  —  the  rise  in  pressure  in  great  measure 
corresponding  to  the  phase  of  inspiration,  the  fall  in  pressure 
to  the  phase  of  expiration.  This  statement  is  not  quite 
accurate,  as  will  be  seen  when  considering  the  influence  of 
respiration  on  circulation  (see  page  295).  These  variations 


I        I         I 

FlO.  123.  —  Tracing  of  Arterial  Blood  Pressure  to  show  large  Respiratory  Varia- 
tions, and  small  Variations  due  to  Heart  Beats,  a  to  6,  Inspiration  ;  6  to  a1, 
Expiration. 

are  easily  seen  in  a  tracing  of  the  arterial  pressure  taken  with 
the  mercurial  manometer  (Fig.  123). 

A  pulse  synchronous  with  the  respirations  may  also  be 
observed  in  the  great  veins  at  the  root  of  the  neck  and  in 
the  cranium  when  opened.  With  each  inspiration  they  tend 
to  collapse,  with  each  expiration  they  again  expand.  The 
reason  for  this  is  that  during  inspiration  the  pressure  inside 
the  thorax  becomes  low  and  hence  blood  is  sucked  from  the 
veins  into  the  heart,  while  during  expiration  the  intra-thoracic 
pressure  becomes  higher  and  thus  the  entrance  of  blood  into 
the  heart  is  opposed. 


THE  CIRCULATION 


257 


3.  Mean  Blood  Pressure. 

I.  GENERAL   DISTRIBUTION. 

That  the  pressure  is  positive — greater  than  the  pressure  of 
the  atmosphere — throughout  the  greater  part  of  the  blood 
vessels  is  indicated  by  the  fact  that  if  a  vessel  is  opened,  the 
blood  flows  out  of  it.  The  force  with  which  blood  escapes  is 
a  measure  of  the  pressure  in  that  particular  vessel.  If  an 


B 

FIG.  124. — A,  The  Mercurial  Manometer  with  recording  float,  used  in  taking 
records  of  the  arterial  blood  pressure  of  lower  animals.  B,  The  Hill- Barnard 
Spbygmometer,  for  measuring  the  arterial  pressure  in  man. 


artery  be  cut,  the  blood  escapes  with  great  force  ;  if  a  vein  be 
cut,  with  much  less  force. 

Actually  to  measure  the  pressure  in  arteries  and  veins  in 
the  lower  animals  is  easy.  It  is  only  necessary  to  let  the 
escaping  blood  act  against  some  measured  force — e.g.  the 
force  of  gravity,  or  a  column  of  mercury.  The  instru- 
ment most  frequently  employed  is  a  U  tube  containing 
mercury,  one  limb  of  which  is  connected  with  an  artery  by 

17 


258 


HUMAN   PHYSIOLOGY 


means  of  a  rigid-walled  tube  filled  with  some  fluid  to  pre- 
vent coagulation.  Before  starting  an  observation  the  pres- 
sure in  this  tube  is  raised  to  something  like  that  expected 
in  the  artery,  and  thus  a  rush  of  blood  into  the  tube  is 
prevented.  (Fig.  124,  A.) 

In  man  it  may  be  done  by  taking  advantage  of  the  fact 
that,  when  the  pressures  inside  and  outside  an  artery  are 
equal,  the  pulse  wave  causes  the  greatest  variation  in  the 
size  of  the  artery.  This  may  be  determined  by  Barnard  and 
Hill's  Sphygmometer,  which  is  made  on  the  principle  of  an 
anseroid  barometer  attached  to  an  elastic  sac  placed  over  an 
artery.  As  the  pressure  is  increased  in  this  system,  the 
pulse  of  the  artery,  as  shown  by  the  hand  of  the  android, 
becomes  more  and  more  marked,  until  it  reaches  a  maximum, 
when  the  pressure  in  the  sac  is  equal  to  that  in  the  artery. 
(Fig  124,  A) 

Arteries. — If  the  pressure  in  the  aorta,  in  the  radial,  in 
the  dorsalis  pedis,  and  in  one  of  the  smallest  arteries  is 

measured,  it    is 
V.  found      that 

while  it  is  great 
in  the  great 
arteries —  about 
160  mm.  Hg  in 
the  aorta — it  is 
much  less  in  the 
small  arteries. 
This  distribu- 
tion of  arterial 
pressure  might 
.  be  plotted  out  as 


160 


Ar 


20 


FlG.  125.— Diagram  of  the  Distribution  of  Mean  Blood  Pres-   in  Fig.  125,  Ar. 

sure  throughout  the  Blood  Vessels.     Ar.,  the  Arteries  ;         Vftins If 

C.,  the  Capillaries ;    V.,  the  Veins. 

the  pressure  in 

any  of  the  small  veins,  in  a  medium  vein,  and  in  a  large 
vein  near  the  heart  be  measured,  it  will  be  found— 

1st.  That  the  venous  pressure  is  less  than  the  lowest 
arterial  pressure. 

2nd.  That  it  is  highest  in  the  small  veins,  and  becomes 
lower  in  the  larger  veins.  In  the  great  veins  entering  the 


THE  CIRCULATION  259 

heart  it  is  lower  than  the  atmospheric  pressure  during  the 
first  part  of  each  ventricular  diastole.     (Fig.  125,  V.) 

Capillaries. — The  pressure  in  the  capillaries  must  obviously 
be  intermediate  between  that  in  the  arteries  and  in  the  veins. 
It  is  not  so  easily  measured,  but  it  may  be  approximately 
arrived  at  by  finding  the  pressure  which  is  required  to  empty 
the  capillaries — e.g.  to  blanch  a  piece  of  skin. 


II.  ARTERIAL  PRESSURE. 

The  force  of  the  heart  and  the  degree  of  peripheral  resist- 
ance both  modify  the  arterial  pressure,  and  normally  these  so 
act  together  that  disturbance  of  one  is  compensated  for  by 
changes  in  the  other.  Thus,  if  the  heart's  action  becomes 
increased  and  tends  to  raise  the  arterial  pressure,  the  peri- 
pheral resistance  falls  and  prevents  any  marked  rise. 
Similarly,  if  the  peripheral  resistance  is  increased,  the 
heart's  actidn  is  diminished,  and  no  rise  in  the  pressure 
occurs.  Under  certain  conditions,  however,  this  compen- 
satory action  is  not  complete,  and  changes  in  the  arterial 
pressure  are  thus  brought  about. 

The  volume  of  blood  has  a  comparatively  small  influence 
on  the  arterial  pressure  because  the  veins  are  so  large  that 
they  accommodate  very  varying  amounts  of  fluid. 

Factors  controlling  Arterial  Pressure. 

(a)  Heart's  action. — The  influence  of  this  may  be  readily 
demonstrated  by  stimulating  the  vagus  nerve  while  taking  a 
tracing  of  the  arterial  pressure.  The  heart  is  inhibited, 
less  blood  is  forced  into  the  arteries,  and  the  pressure 
falls. 

If,  on  the  other  hand,  the  accelerator  nerve  is  stimulated, 
the  increased  heart's  action  drives  more  blood  into  the 
arteries,  and  the  pressure  rises. 

(6)  Peripheral  resistance. — The  resistance  to  outflow 
from  the  arteries  depends  upon  the  resistance  offered  in  the 
small  arteries,  the  walls  of  which  are  chiefly  composed  of 
muscular  tissue.  When  these  fibres  are  contracted,  the 
lumen  of  the  vessels  is  small  and  the  resistance  is  great. 


260  HUMAN   PHYSIOLOGY 

When  it  is  relaxed,  the  lumen  of  the  vessels  dilates,  and  the 
resistance  to  outflow  is  diminished.  This  muscular  tissue 
of  the  arterioles  acts  as  a  stop-cock  to  the  flow  of  blood 
from  the  arteries  to  the  capillaries.  It  is  of  great  import- 
ance— 

1st.  In  maintaining  the  uniform  pressure  in  the  arteries. 

2nd.  In  regulating  the  flow  of  blood  into  the  capillaries. 

During  the  functional  activity  of  a  part,  a  free  supply  of 
blood  in  its  capillaries  is  required.  This  is  brought  about 
by  a  relaxation  of  the  muscular  coats  of  the  arterioles  leading 
to  the  part.  When  the  part  returns  to  rest,  the  free  flow  of 
blood  is  checked  by  the  contraction  of  the  muscular  walls  of 
the  arterioles. 

The  action  of  the  arterioles  is  well  seen  under  the  influence 
of  certain  drugs  (vaso-dilators  and  vaso-constrictors).  If, 
while  a  tracing  of  the  arterial  pressure  is  being  taken,  nitrite 
of  amyl  is  administered  to  the  animal,  it  will  be  seen  that  the 
skin  and  mucous  membranes  become  red  and  engorged  with 
blood,  while  at  the  same  time  the  arterial  pressure  falls. 
Nitrites  cause  the  muscular  coat  of  the  arterioles  to  relax, 
and  thus,  by  diminishing  peripheral  resistance,  permit  blood 
to  flow  freely  from  the  arteries  into  the  capillaries. 

Salts  of  barium  have  precisely  the  opposite  effect,  causing 
the  skin  to  become  pale  from  imperfect  filling  of  the  capil- 
laries, and  producing  a  marked  rise  in  the  arterial  pressure. 
Contraction  of  the  muscles  of  the  arterioles  is  produced,  and 
the  flow  of  blood  from  arteries  to  capillaries  is  retarded. 

Not  only  is  the  state  of  the  arterioles  influenced  thus  by 
drugs,  but  it  is  also  affected  by  the  internal  secretions  (see 
p.  284)  from  certain  organs.  In  all  probability  a  vaso-dilator 
substance  is  formed  in  the  thyroid,  while  a  powerful  vaso- 
constrictor is  certainly  produced  in  the  medulla  of  the  supra- 
renals. 

The  condition  of  the  arterioles  may  be  studied  in  many 
different  ways — 

1st.  By  direct  observation.  1.  With  the  naked  eye.  A  red 
engorged  appearance  of  any  part  of  the  body  may  be  due  to 
dilatation  of  the  arteriole  leading  to  it.  It  may,  however,  be 
due  to  some  obstruction  to  the  outflow  of  blood  from  the  part. 
2.  With  the  microscope.  In  certain  transparent  structures, 


THE   CIRCULATION  261 

such  as  the  web  of  the  frog's  foot,  or  the  wing  of  the  bat,  or 
the  mesentery,  it  is  possible  to  measure  the  diameter  of  the 
arterioles  by  means  of  an  eye-piece  micrometer,  and  to  study 
their  dilatation  and  contraction. 

2nd.  The  engorgement  of  the  capillaries  brought  about  by 
dilatation  of  the  arterioles  manifests  itself  also  in  an  increased 
size  of  the  part.  Every  one  knows  how  on  a  hot  day,  when 
the  arterioles  of  the  skin  are  dilated,  it  is  difficult  to  pull  on 
a  glove,  which,  on  a  cold  day,  when  the  cutaneous  arterioles 
are  contracted,  feels  loose.  By  enclosing  a  part  of  the  body 
in  a  case  with  rigid  walls  filled  with  fluid  or  with  air  which  is 
connected  with  some  form  of  recording  tambour,  an  increase 
or  decrease  in  the  size  of  the  part  due  to  the  state  of  its 
vessels  may  be  registered.  Such  an  instrument  is  called  a 
plethysmograph. 

3rd.  When  the  arterioles  to  a  part  are  dilated  and  the 
blood  is  flowing  freely  into  the  capillaries,  the  part  becomes 
warmer,  and  by  fixing  a  thermometer  to  the  surface  conclu- 
sions as  to  the  condition  of  the  arterioles  may  be  drawn. 

4tth.  By  streaming  blood  through  the  vessels  and  observing 
the  rate  at  which  it  escapes  the  changes  in  the  state  of  the 
arterioles  may  be  made  out.  This  perfusion  method  is  much 
used  in  studying  the  action  of  drugs.  (Practical  Physiology, 
Chap.  X.) 

5th.  Since  the  state  of  the  arterioles  influences  the  arterial 
pressure,  if  the  heart's  action  is  kept  uniform,  changes  in  the 
arterial  blood  pressure  indicate  changes  in  the  arterioles, 
a  fall  of  pressure  indicating  dilatation,  a  rise  of  pressure, 
constriction. 

Normal  State  of  Arterioles. — If  an  arteriole  in  some 
transparent  tissue  be  examined,  it  will  be  found  to  main- 
tain a  fairly  uniform  size,  but  to  undergo  periodic  slow 
changes  in  calibre.  If  the  ear  of  a  white  rabbit  be  studied, 
it  will  be  seen  to  undergo  slow  changes,  at  one  time  appear- 
ing pale  and  bloodless,  at  another  time  red  and  engorged. 
During  this  latter  phase  numerous  vessels  appear,  which  in 
the  former  condition  were  invisible.  These  slow  changes 
are  independent  of  the  heart's  action  and  of  the  rate  of 
respiration.  They  appear  to  be  due  to  the  periodic  rhythmic 
contraction  which  is  a  characteristic  property  of  non-striped 


262  HUMAN  PHYSIOLOGY 

muscle  fibres.     This  rhythmic  action  is  better  marked  in 
some  vessels  than  in  others. 

Yaso-motor  Mechanism. — If  the  sciatic  nerve  of  a  frog  be 
cut,  the  arterioles  in  the  foot  at  once  dilate.  If  the  sciatic  is 
stimulated,  the  arterioles  become  smaller.  The  same  results 
follow  if  the  anterior  roots  of  the  lower  spinal  nerves,  from 
which  the  sciatic  takes  origin,  be  cut  or  stimulated. 

We  must,  therefore,  conclude  (1)  that  the  central  nervous 
system  exerts  a  constant  tonic  influence  upon  the  arterioles, 
keeping  them  in  a  state  of  semi-contraction ;  and  (2)  that  this 
influence  may  be  increased,  and  thus  a  constriction  of  the 
arterioles  caused,  and  in  this  way  the  flow  of  blood  from 
arteries  to  capillaries  obstructed  and  the  arterial  pressure 
raised ;  and  (3)  that  this  influence  may  be  diminished,  so 
that  the  arterioles  dilate  and  allow  an  increased  flow  into  the 
capillaries  from  the  arteries,  and  thus  lower  the  arterial 
pressure. 

These  mobile  arterioles,  under  the  control  of  the  central 
nervous  system,  constitute  a  vaso-motor  mechanism,  which 
plays  a  most  important  part  in  connection  with  nearly  every 
vital  process  in  the  body.  By  it  the  pressure  in  the  arteries 
is  governed,  by  it  the  supply  of  blood  to  the  capillaries  and 
tissues  is  controlled,  and  by  it  the  loss  of  heat  from  the  body 
is  largely  regulated. 

This  vaso-motor  mechanism  consists  of  the  three  parts : — 

1st.  The  contractile  muscular  walls  of  the  arterioles  with 
the  nerve  terminations  in  them. 

2nd.  The  nerves  which  pass  to  them. 

3rd.  The  portions  of  the  central  nervous  system  presiding 
over  these. 

1.  Muscular  Walls  of  the  Arterioles. — The  muscular  fibres 
are  maintained  in  a  state  of  tonic  semi-contraction  by  nerves 
passing  to  them,  and  when  these  nerves  are  divided,  the 
muscular  fibres  relax.  But  if,  after  these  nerves  have  been 
cut,  the  animal  be  allowed  to  live,  in  a  few  days  the  arterioles 
again  pass  into  a  state  of  tonic  semi-contraction,  although 
no  union  of  the  divided  nerve  has  taken  place. 

Certain  drugs,  e.g.  digitalis  and  the  salts  of  barium,  act  as 
direct  stimulants  to  these  muscle  fibres. 


THE  CIRCULATION 


263 


It   appears   that   the   muscular  fibres  in   the   arterioles, 
as    elsewhere,    tend    to    maintain    themselves   in    a    state 
of    partial    contraction,    which    increases    and    diminishes 
in    a    regular    rhythmic 
manner. 

The  precise  part  played 
by  the  nerve  terminations 
has  not  been  definitely  estab- 
lished, but  certain  drugs 
appear  to  act  specially  upon 
them.  Thus  apocodeine, 
while  it  does  not  prevent 
barium  salts  from  constrict- 
ing the  vessels,  prevents  the 
constricting  action  of  ex- 
tracts of  the  medulla  of 
the  suprarenals,  even  when 
the  nerves  are  cut ;  and 
hence  it  must  be  concluded 
that  it  paralyses  a  nervous 
mechanism  in  the  arteriole 
wall,  which  is  stimulated 
by  the  suprarenal  extract. 

Normally  this  muscular 
mechanism  is  controlled  by 
the  nervous  system. 


2.  Yaso-motor   Nerves.— 

When  a  nerve  going  to  any  FlG.  i26.-Diagram  of  the  Distribution  of 
part  of  the  body  is  cut  the 
arterioles  of  the  part  gene- 
rally dilate,  when  it  is  stimu- 
lated the  arterioles  are 
usually  contracted ;  some- 
times, however,  they  are 
dilated.  In  no  case  does  section  of  a  nerve  cause  constric- 
tion of  the  arterioles. 

These   facts   prove   that   the  vaso-motor  nerves  may  be 
divided  into  two  classes: — 

1st.  Vaso-constrictor. 


Vaso-motor  Nerves.  The  continuous 
line  shows  the  Vaso-constrictors ;  the 
dotted  line  the  Vaso-dilators.  C.N., 
Cranial  Nerves;  Vag.,  Vagus;  T.S., 
Thoracic  Sympathetic ;  A. St.,  Ab- 
dominal Sympathetic;  N.L.,  Nerves 
to  the  Leg. 


264  HUMAN   PHYSIOLOGY 

2nd.  Vaso-dilator. 

A.  Yaso-constrictor    Nerves. — The  fact   that  section   of 
these  at  once  causes  a  dilatation  of  the  arterioles  proves 
that   they  are  constantly  transmitting  impulses  from   the 
central  nervous  system. 

Course. — The  course  of  these  fibres  has  been  investigated 
by  section  and  by  stimulation  (Fig.  126). 

They  leave  the  spinal  cord  chiefly  in  the  dorsal  region 
by  the  anterior  roots  of  the  spinal  nerves,  pass  into  the 
sympathetic  ganglia,  where  they  have  their  cell  stations, 
and  then  as  non-medullated  fibres  pass,  either  along,  the 
various  sympathetic  nerves  to  the  viscera,  or  back  through 
the  grey  ramus  (see  Fig.  75,  p.  145)  into  the  spinal  nerve, 
and  run  in  it  to  their  terminations. 

B.  Yaso-dilator  Nerves. — A  good  example  of  such  a  nerve 
is  to  be  found  in  the  chorda  tympani  branch  of  the  facial 
nerve,  which  sends  fibres  to  the  submaxillary  and  sublingual 
salivary  glands.     If  this  nerve  is  cut,  no  change  takes  place 
in  the  vessels  of  the  gland,  but,  when  it  is  stimulated,  the 
arterioles  dilate  and  allow  an  increased  flow  of  blood  through 
the  capillaries.     These  fibres,  therefore,  instead  of  increasing 
the  activity  of  muscular  contraction,  diminish  or  inhibit  it. 
They  play  the  same  part  in  regard  to  the  muscular  fibres  of 
the  arterioles  as  the  inferior  cardiac  branch  of  the  vagus 
does  in  regard  to  the  cardiac  muscular  fibres.     As  examples 
of  vaso-dilator  nerves  we  may  take  the  gastric  branches  of 
the  vagus  carrying  vaso-dilator  fibres  to  the  mucous  mem- 
brane of  the  stomach,  and  the  nervi  erigentes  carrying  vaso- 
dilator fibres  to  the  external  genitals. 

The  vaso-dilator  nerves  of  most  parts  of  the  body  run  side 
by  side  with  the  vaso-constrictor  nerves ;  and,  hence,  curious 
results  are  often  obtained.  If  the  sciatic  nerve  of  a  dog  is 
cut,  the  arterioles  of  the  foot  dilate.  If  the  peripheral  end 
of  the  cut  nerve  is  stimulated,  the  vessels  contract.  But, 
after  a  few  days,  if  the  nerve  be  prevented  from  uniting,  the 
arterioles  of  the  foot  recover  their  tonic  contraction,  and  if 
the  sciatic  nerve  is  then  stimulated,  a  dilatation,  and  not  a 
constriction,  is  brought  about.  The  vaso-constrictor  fibres 
seem  to  die  more  rapidly  than  the  vaso-dilator  fibres  which 
run  alongside  of  them.  Under  certain  conditions  the  activity 


THE   CIRCULATION  265 

of  the  vasodilator  fibres  seems  to  be  increased.  Thus,  if, 
when  the  lirnb  is  warm,  the  sciatic  nerve  is  stimulated, 
dilatation  rather  than  constriction  may  occur.  Again,  while 
rapidly  repeated  and  strong  induction  shocks  are  apt  to 
cause  constriction,  slower  and  weaker  stimuli  tend  to  pro- 
duce dilatation. 

Course. — The  vaso-dilator  nerves  pass  out  by  the  anterior 
roots  of  the  various  spinal  nerves,  and  do  not  pass  through 
the  sympathetic  ganglia,  but  run  as  medullated  fibres  to 
their  terminal  ganglia  (Fig.  126). 

3.  Portions  of  Nervous  System  Presiding  over  the  Yaso- 
motor  Mechanism. — Since  a  set  of  nerves  causing  constriction 
of  the  arterioles,  and  another  set  causing  dilatation  exist, 
we  must  conclude  that  there  are  two  mechanisms  in  the 
central  nervous  system,  one  a  vaso-constrictor,  the  other  a 
vaso-dilator. 

A.  Yaso- constrict  or  Centre. — (a)  Mode  of  Action. — This 
mechanism  is  constantly  in  action,  maintaining  the  tonic 
contraction  of  the  arterioles. 

If  any  afferent  nerve  be  stimulated,  the  effect  is  to  increase 
the  activity  of  the  mechanism,  to  cause  a  general  constriction 
of  arterioles,  and  thus  to  raise  the  general  arterial  pressure. 
It  is,  therefore,  capable  of  reflex  excitation.  In  ordinary 
conditions  so  many  afferent  nerves  are  constantly  being 
stimulated,  that  it  is  not  easy  to  say  how  far  the  tonic  action 
of  this  centre  is  reflex  and  dependent  on  the  stream  of 
afferent  impulses. 

But  this  centre  may  also  be  directly  acted  upon  by  the 
condition  of  the  blood  and  lymph  circulating  through  it. 
When  the  blood  is  not  properly  oxygenated,  as  in  asphyxia 
or  suffocation,  this  centre  is  stimulated  and  a  general  con- 
striction of  arterioles  with  high  blood  pressure  results. 

(6)  Position. — In  investigating  the  position  of  the  centre 
we  may  take  advantage  of— 

1st.  Its  constant  tonic  influence.  Removal  of  the  centre 
at  once  causes  dilatation  of  arterioles. 

2nd.  The  fact  that  it  may  be  reflexly  stimulated.  If  the 
vaso-constrictor  centre  be  removed,  stimulation  of  an  afferent 
nerve  no  longer  causes  constriction  of  the  arterioles. 


266  HUMAN  PHYSIOLOGY 

Removal  of  the  whole  brain  above  the  pons  Varolii  leaves 
the  centre  intact. 

Separation  of  the  pons  Varolii  and  medulla  oblongata 
from  the  spinal  cord  at  once  causes  a  dilatation  of  the 
arterioles  of  the  body,  and  at  once  prevents  the  produc- 
tion of  reflex  constriction  by  stimulation  of  an  afferent 
nerve. 

The  main  part,  at  least,  of  the  vaso-constrictor  mechanism 
is  therefore  situated  in  the  pons  Varolii  and  medulla 
oblongata. 

The  extent  of  this  centre  has  been  determined  by  slicing 
away  this  part  of  the  brain  from  above  downwards,  and 
studying  the  influence  of  reflex  stimulation  after  the  removal 
of  each  slice. 

It  is  found  that  at  a  short  distance  below  the  corpora  quad- 
rigemina,  the  removal  of  each  succeeding  part  is  followed  by 
a  diminution  in  the  reflex  constriction,  until,  at  a  point  close 
to  and  just  above  the  calamus  scriptorius,  all  reflex  response 
to  stimulation  stops. 

The  centre  is  therefore  one  of  very  considerable  longi- 
tudinal extent. 

But  it  has  been  found  that,  if,  after  section  of  the  spinal 
cord  high  up,  the  animal  be  kept  alive  for  some  days,  the 
dilated  arterioles  again  contract,  and  stimulation  of  afferent 
nerves  entering  the  cord  below  the  point  of  section  causes 
a  further  constriction.  If  now  another  section  be  made 
further  down  the  cord,  the  arterioles  supplied  by  nerves 
coming  from  below  the  point  of  section  will  again  dilate. 
This  shows  that  secondary  vaso-constrictor  centres,  tonic 
in  action  and  capable  of  having  their  activity  reflexly 
increased,  exist  all  down  the  grey  matter  of  the  spinal  cord. 
Normally  these  are  under  the  domain  of  the  dominant 
centre,  but  when  this  is  out  of  action  they  then  come  into 
play. 

B.  Yaso-dilator  Centre. —  (a)  Mode  of  Action.— This 
mechanism  is  not  constantly  in  action.  Section  of  a  vaso- 
dilator nerve  does  not  cause  vascular  dilatation.  It  may  be 
excited  reflexly,  but  in  a  different  manner  from  the  vaso- 
constrictor mechanism. 

Stimulation  of  an  afferent  nerve  causes  a  dilatation  of  the 


THE   CIRCULATION  267 

arterioles  in  the  part  from  which  it  comes,  and  a  constric- 
tion of  the  arterioles  throughout  the  rest  of  the  body.  If 
a  sapid  substance,  such  as  pepper,  be  put  in  the  mouth, 
the  buccal  mucous  membrane  and  the  salivary  glands 
become  engorged,  while  there  is  a  constriction  of  the 
arterioles  throughout  the  body.  The  vaso-dilator  central 
mechanism  is  not  general  in  its  action  like  the  vaso- 
constrictor, but  is  specially  related  to  the  different  parts  of 
the  body. 

This  is  a  matter  of  the  greatest  importance  in  physiology 
and  pathology.  It  explains  the  increased  vascularity  of  a 
part  when  active  growth  is  going  on.  The  changes  in  the  part, 
or  the  products  of  these,  stimulate  the  afferent  nerve.  This 
reflexly  stimulates  the  vaso-dilator  mechanism  of  the  part, 
and  thus  causes  a  free  flow  of  blood  into  the  capillaries,  and 
at  the  same  time,  by  causing  a  general  constriction  of  the 
arterioles,  maintains  or  actually  raises  the  arterial  pressure, 
and  thus  forces  more  blood  to  the  situation  in  which  it  is 
required. 

The  same  process  occurs  in  the  case  of  the  stomach  during 
digestion,  in  the  case  of  the  kidney  during  secretion,  and 
in  the  process  of  inflammation. 

Not  only  does  peripheral  stimulation  act  in  this  way,  but 
various  states  of  the  brain,  accompanied  by  emotions,  may 
stimulate  part  of  the  vaso-dilator  mechanism,  as  in  the  act 
of  blushing. 

Again,  it  has  been  shown  that  stimulation  of  the  central 
end  of  the  depressor  nerve  (superior  cardiac  branch  of  the 
vagus)  causes  a  dilatation  of  the  arterioles  chiefly  in  the 
abdominal  cavity,  but  also  throughout  the  body  generally. 
This  is  the  most  generalised  vaso-dilator  reflex  known 
(see  p.  240). 

(b)  Position. — While  the  dominant  vaso-constrictor  centre 
is  in  the  medulla,  the  vaso-dilator  centres  seem  to  be  dis- 
tributed throughout  the  medulla  and  spinal  cord. 

III.  CAPILLARY  PRESSURE. 

It  has  already  been  shown  that  this  is  less  than  in  the 
arteries  and  greater  than  in  the  veins. 


268  HUMAN  PHYSIOLOGY 

Like  the  pressure  in  the  arteries  it  depends  upon  the  two 
factors — 

1st.  Force  of  inflow. 

2nd.  Resistance  to  outflow. 

(a)  Variations  in  the  Force  of  Inflow. — The  capillary 
pressure  may  undergo  marked  local  changes  through  the 
vaso-motor  mechanism.  Wherever  the  function  of  a  part  is 
active,  dilatation  of  the  arterioles  and  an  increased  capillary 
pressure  exists,  and,  when  the  influence  of  vaso-dilator  nerves 
is  withdrawn,  the  capillary  pressure  falls. 

But  the  capillary  pressure  may  also  be  modified  by 
the  heart's  action,  inasmuch  as  the  arterial  pressure,  by 
which  blood  is  driven  into  the  capillaries,  depends  upon 
this.  In  cardiac  inhibition  not  only  is  arterial  pressure 
lowered,  but  capillary  pressure  may  also  fall.  In  aug- 
mented heart  action  both  arterial  and  capillary  pressure 
are  raised. 

(6)  Variations  in  Resistance  to  Outflow. — Normally  the 
flow  from  capillaries  to  veins  is  free  and  unobstructed ;  but, 
if  the  veins  get  blocked,  or  if  the  flow  in  them  is  retarded  by 
gravity,  the  capillaries  get  engorged  with  the  blood  which 
cannot  escape  from  them.  This  increased  pressure  in  the 
capillaries  is  very  different  from  that  caused  by  increased 
inflow.  The  flow  through  the  vessels  is  slowed  or  may 
be  stopped  instead  of  being  accelerated,  and  the  blood  gets 
deprived  of  its  nourishing  constituents,  loaded  with  waste 
products,  and  tends  to  exude  into  the  lymph  spaces,  causing 
dropsy. 

It  is,  therefore,  most  important  to  distinguish  between 
high  capillary  pressure  from  dilated  arterioles  or  an 
active  heart,  and  high  pressure  due  to  venous  obstruc- 
tion. 

A  condition  very  similar  to  that  described,  but  producing 
a  capillary  pressure  high  relatively  to  the  pressure  in  the 
arteries — though  not  absolutely  high — is  seen  in  cases  of 
failure  of  the  heart,  when  that  organ  is  not  acting  sufficiently 
strongly  to  pass  the  blood  on  from  the  venous  into  the 
arterial  system.  Here  the  arterial  pressure  becomes  lower 
and  lower,  the  venous  pressure  higher  and  higher,  and  along 
with  this  the  capillary  pressure  becomes  high  in  relationship 


THE   CIRCULATION 


269 


to  the  arterial  pressure ;  the  blood  is  not  forced  through  these 
channels,  and  congestion  of  the  capillaries  and  dropsy  may 
result. 

The  influence  of  gravity  plays  a  very  important  part  on 
the  capillary  pressure,  since  it  has  so  marked  an  influence 
on    the    flow   of   blood    in 
the  veins.     When,  through 
heart  failure,  the   blood   is 
not     properly     sucked     up 
from   the  inferior  extremi- 
ties, this  increased  pressure 
becomes   very  marked    in- 
deed. 

But  the  pressure  in  the 
capillaries  may  also  to  a 
certain  extent  be  varied  by 
the  withdrawal  of  water 

from   the    body,   as    in     pur-      Fia  127.— The  Changes  in  Blood  Pressure 
,.  -,.  .  in  the  Capillaries  produced  by  increas- 

gation    or    diuresis,  or    by          ing  the  ^rterial  ;ressure  .5 , 

the    addition  of  large  quan-  and    by   obstructing  the  venous  flow 

tities  of  fluid  to  the  blood.          •  7f  •  Arteries;  &•  Cnpii- 

.,,,  .      ,  lanes ;   V. ,  Veins. 

I  he  venous  system  is,  how- 
ever, so  capacious  that  very  great  changes  in  the  amount 
of  blood  in  the  vessels  may  take  place  without  materially 
modifying  the  arterial  or  capillary  pressure  while  affecting 
temporarily  the  venous  pressure. 


IV.  VENOUS  PEESSURE. 

In  the  veins  the  force  of  inflow  is  small;  the  resistance 
to  outflow  is  nil.  Hence  the  pressure  is  small,  and  steadily 
diminishes  from  the  small  veins  to  the  large  veins  entering 
the  heart  (Fig.  127). 

The  venous  pressure  may  be  modified  by  variations  in 
these  two  factors.  Constriction  of  the  arterioles  tends  to 
lower  the  venous  pressure,  dilatation  to  raise  it.  On  the 
other  hand,  increased  heart's  action,  which  so  markedly 
tends  to  raise  arterial  pressure,  diminishes  the  pressure  in 
the  larger  veins,  because  the  blood  is  thus  more  rapidly 
driven  from  veins  into  arteries,  and  because  the  heart,  which 


270  HUMAN   PHYSIOLOGY 

in  its  powerful  systole  drives  out  more  blood,  in  its  diastole 
sucks  in  more. 

Compression  of  the  thorax  has  a  very  marked  effect  in 
retarding  the  flow  of  blood  from  the  great  veins  into  the 
heart,  and  thus  tends  to  raise  the  venous  pressure  and  to 
lower  the  arterial  pressure.  Venous  pressure  may  be  tem- 
porarily modified  by  the  loss  or  gain  of  water. 

V.  LYMPHATIC  PRESSURE. 

No  exact  determination  of  the  lymph  pressure  in  the 
tissue  spaces  has  been  made,  but  since  there  is  a  constant 
flow  from  these  spaces  through  the  lymphatic  vessels  and 
through  the  thoracic  duct  into  the  veins  at  the  root  of  the 
neck,  the  pressure  in  the  tissue  spaces  must  be  higher  than 
the  pressure  in  the  great  veins. 

This  pressure  is  kept  up  by  the  formation  of  lymph  from 
the  blood,  and  from  the  cells  of  the  tissues  (see  p.  207). 


B.— Flow  of  Blood. 

The  flow  of  blood,  as  already  indicated,  depends  upon  the 
distribution  of  pressure,  a  fluid  always  tending  to  flow  from 
the  point  of  higher  pressure  to  the  point  of  lower  pressure. 
Since  a  high  pressure  is  maintained  in  the  aorta  and  a  low 
pressure  in  the  veins  entering  the  heart  and  in  the  cavities 
of  the  heart  during  its  diastole,  the  blood  must  flow  through 
the  circulation  from  arteries  to  veins. 

The  velocity  of  the  flow  of  a  fluid  depends  upon  the 
width  of  the  channel.  Since  in  unit  of  time  unit  of 
volume  must  pass  each  point  in  a  stream  if  the  fluid  is 
not  to  accumulate  at  one  point,  the  velocity  must  vary 
with  the  sectional  area  of  the  channel.  In  the  case  of  a 
river,  in  each  second  the  same  amount  of  water  must  pass 
through  the  narrowest  and  through  the  widest  part  of  its 
channel.  Now  for  a  ton  of  water  to  get  through  any  point 
in  a  channel  one  square  yard  in  sectional  area  in  the  same 
time  as  it  takes  to  pass  a  point  in  a  channel  ten  square  yards 
in  area,  it  must  obviously  flow  with  greater  velocity.  This 
may  be  stated  in  the  proposition  that  the  velocity  (V) 


THE   CIRCULATION 


271 


of  the  stream  is  equal  to  the  amount  of  blood  passing  any 
point  per  second  (v)  divided  by  the  sectional  area  of  the 
stream  (S) — 


where    S    is    the   radius   squared    multiplied   by   the    con- 
stant 314. 

In  the  vascular  system  the  sectional  area  of  the  aorta 
is   small   when   compared   with   the   sectional   area   of  the 


AR 


FlG.  128. — Diagram  of  the  Sectional  Area  of  the  Vascular  System,  upon 
which  the  Velocity  of  the  Flow  depends.  AR.,  Arteries;  (7., 
Capillaries  ;  V.,  Veins. 

smaller  arteries;  while  the  sectional  area  of  the  capillary 
system  is  no  less  than  700  times  greater  than  that  of  the  aorta. 
In  the  venous  system  the  sectional  area  steadily  diminishes, 
although  it  never  becomes  so  small  as  in  the  corresponding 
arteries,  and  where  the  great  veins  enter  the  heart  it  is  about 
twice  the  sectional  area  of  the  aorta  (Fig.  128). 

This  arrangement  of  the  sectional  area  of  the  stream  gives 
rise  to  a  rapid  flow  in  the  arteries,  a  somewhat  slower  flow  in 
the  veins,  and  to  a  very  slow  flow  in  the  capillaries. 

The  suddenness  of  the  change  of  pressure  has  a  certain 
influence  on  the  rapidity  of  flow,  as  is  well  seen  in  a  river. 
If  the  water  descends  over  a  sudden  declivity  to  a  lower  level 
it  attains  a  much  greater  velocity  than  if  the  declivity  is 


272  HUMAN  PHYSIOLOGY 

gentle.  In  the  first  case  the  change  of  pressure  is  sudden, 
in  the  second  case  it  is  slow. 

Hence,  if  from  any  cause  the  pressure  is  raised  at  any 
point,  the  flow  will  tend  to  be  more  rapid  from  that  point 
onwards  till  the  normal  distribution  of  pressure  is  re- 
established. 

Friction  has  also  a  certain  effect.  A  river  runs  much 
faster  in  mid-stream  than  along  the  margins,  because  near 
the  banks  the  flow  is  delayed  by  friction,  and  the  more 
broken  up  and  subdivided  is  the  channel,  the  greater  is 
the  friction  and  the  more  is  the  stream  slowed. 

When,  therefore,  in  the  capillary  system,  the  blood  stream 
is  distributed  through  innumerable  small  channels,  the 
friction  is  very  great,  and  this  tends  to  dam  back  the 
blood. 

The  velocity  of  flow  in  the  arteries  and  veins  may  be 
measured  by  various  methods,  of  which  one  of  the  best  is 
that  by  means  of  the  stromuhr,  an  instrument  by  which 
the  volume  of  blood  passing  a  given  point  in  an  artery  or 
vein  in  a  given  time  may  be  determined.  The  velocity  of 
the  flow  in  the  capillaries  may  be  measured  in  transparent 
structures  by  means  of  a  microscope  with  an  eye-piece 
micrometer.  (Practical  Exercise.}  The  velocity  of  the 
blood  is— 

Carotid  of  the  dog  about   .        .     300  mm.  per  sec. 
Capillaries  about        .         .         .     0-5  to  1  m.    „ 
Vein  (jugular)  about         .         .150  mm.       „ 

It  is  not  so  easy  to  give  definite  figures  for  the  velocity  of 
the  lymph  stream. 

Disturbance  of  any  of  the  factors  which  govern  the  rate 
of  flow  will  bring  about  alterations  in  the  velocity  of  the 
blood  in  arteries,  capillaries,  and  veins.  Thus,  an  increased 
venous  pressure,  by  leading  to  a  diminution  in  the  difference 
of  pressure  between  arteries  and  veins,  will  materially  slow  the 
blood  stream.  Great  dilatation  of  the  arterioles  will  slow  the 
blood  stream  in  them ;  and  increased  viscosity  of  the  blood 
by  increasing  friction  with  the  vessel  wall  will  also  slow  the 
stream. 


THE   CIRCULATION  273 

Special  Characters  of  Blood  Flow. 

(a)  Arteries. — The  flow  of  blood  in  an  artery  is  rhythmi- 
cally accelerated  with  each  ventricular  systole.     This  is  due 
to  the  pulse  wave.      As  the  wave  of  high  pressure  passes 
along   the   vessels,   the  blood   tends   to   flow  forwards   and 
backwards   from  it — so  that   in  front  of  the  wave  there  is 
an  acceleration  of  the  stream  and  behind  it  a  retardation. 
In   a   wave   at   sea   the   same   thing   happens,  and   a  cork 
floating  on  the   surface  is  moved   forward  in  front  of  the 
wave  and  again  backwards  after  the  wave  has  passed. 

(b)  Capillaries. — In  the  capillaries  the  flow  is  uniform. 

(c)  Veins. — In  most  veins,  too,  it  is  uniform,  but  in  the 
great  veins  near  the  heart  it  undergoes  periodic  accelera- 
tions— 

1st.  With  each  diastole  of  auricle  and  ventricle. 
2nd.  With  each  inspiration. 

3rd.  By  muscular  action  squeezing  the  blood  out  of  the 
small  veins. 

In  all  vessels  the  blood  in  the  centre  of  the  stream 
moves  more  rapidly  than  that  at  the  periphery  on  account 
of  the  friction  between  the  blood  and  the  vessels.  An 
"axial"  rapid  and  "peripheral"  slow  stream  are,  there- 
fore, described.  This  is  well  seen  in  any  small  vessel  placed 
under  the  microscope,  and  in  such  situations  it  will  be  found 
that,  while  the  erythrocytes  are  chiefly  carried  in  the  axial 
stream,  the  leucocytes  are  more  confined  to  the  peripheral 
stream,  where  they  may  be  observed  to  roll  along  the  vessel 
wall  with  a  tendency  to  adhere  to  it. 

When  from  any  cause  the  flow  through  the  capillaries  is 
brought  to  a  standstill,  these  leucocytes  creep  out  through 
the  vessel  wall  and  invade  the  tissue  spaces.  This  is  the 
process  of  diapedesis,  which  plays  an  important  part  in 
inflammation. 

In  at  least  two  situations  the  circulation  presents  special 
characters. 

1.  Circulation  Inside  the  Cranium. — Here  the  blood  circu- 
lates in  a  closed  cavity  with  rigid  walls,  and  therefore  its 

18 


274  HUMAN   PHYSIOLOGY 


amount  can  vary  only  at  the  expense  of  the  cerebro-spinal 
fluid.  This  is  small  in  amount,  and  permits  of  very  small 
variations  in  the  volume  of  blood.  Increased  arterial  pres- 
sure in  the  body  does  not  therefore  increase  the  amount  of 
blood  in  the  brain,  but  simply  drives  the  blood  more  rapidly 
through  the  organ.  There  seems  to  be  no  regulating  nervous 
mechanism  connected  with  the  arteriolcs  of  the  brain,  and 
the  cerebral  pressure  simply  follows  the  changes  in  the 
general  arterial  pressure.  The  splanchnic  area  is  the  great 
regulator  of  the  supply  of  blood  to  the  brain.  Since  the 
cerebral  arteries  are  supported  and  prevented  from  dis- 
tending by  the  solid  wall  of  the  skull,  the  arterial  pulse 
tends  to  be  propagated  into  the  veins,  and  in  these  veins 
the  respiratory  pulse  is  very  well  marked  (Fig.  129). 

2.  Circulation   in  the   Lungs. — Vaso-motor  nerves  seem 
to  be  absent,  and  hence  drugs  like  adrenalin  fail  to  cause  a 
constriction  of  the  arterioles.     The  amount  of  blood  in  the 
lungs  is  regulated  by  the  blood  pressure  in  the  systemic 
vessels. 

3.  Circulation  in  Heart  Wall. — A  peripheral  vaso-motor 
mechanism  is  not  present  in  the  arterioles  of  the  coronary 
vessels  (see  also  p.  231). 

Extra-Cardiac  Factors  Maintaining  Circulation. 

In  considering  the  flow  of  blood  through  the  vessels,  due 
to  the  distribution  of  pressure  in  arteries  and  veins,  it  must 
be  remembered  that  the  central  pump  or  heart  is  not  the 
only  factor  maintaining  it  (Fig.  129). 

The  thorax  is  to  be  looked  upon  as  a  suction  pump  of 
considerable  power,  which  draws  blood  into  the  heart  during 
inspiration.  Again,  the  abdominal  blood  vessels  are  to  be 
regarded  as  the  great  blood  reservoir,  and  when  the  abdo- 
minal muscles  are  tightened  and  the  respiratory  movements 
of  the  thorax  are  increased,  as  in  the  panting  which  accom- 
panies intermittent  muscular  exercise,  the  blood  is  partly 
pressed,  and  partly  sucked  from  the  abdomen  into  the  heart, 
and  so  forced  on  into  the  arteries.  Even  expiration  helps 
in  this,  for  the  blood  which  has  filled  the  vessels  of  the  lungs 
in  inspiration  is  driven  on  into  the  left  side  of  the  heart  in 


THE   CIRCULATION 


275 


expiration.  The  blood  is  thus  forced  on  into  the  arteries  and 
so  to  the  muscles,  and  they,  by  their  alternate  contraction 
and  relaxation,  still  further  help  to  drive  it  on  and  to  accele- 
rate the*  circulation.  The  high  arterial  tension  tends  to 
drive  the  blood  through  the  cranial  vessels.  The  benefit 


INTRACRANIAL    CIRCULATION 


PULMONARY      CIRCULATION 


[ABDOMINAL   CIRCULATION 


CIRCULATION    IN    LIMBS 


FIG.  129. — Scheme  of  the  Circulation,  modified  from  Hill,  to  illustrate  the  influence 
of  the  various  extra-Cardiac  Factors  which  maintain  the  Flow  of  Blood. 

of  intermittent  muscular  exercise  on  the  circulation  is  thus 
manifest. 

When,  on  the  other  hand,  some  sustained  muscular  strain 
has  to  be  undergone,  the  thorax,  is  fixed  and  hence  the 
pressure  in  the  heart  and  thoracic  organs  is  raised,  and  the 
increased  pressure  in  the  thorax  helps  to  support  the  heart 
and  to  prevent  over-distension.  The  abdominal  vessels  are 


276  HUMAN  PHYSIOLOGY 

also  pressed  upon,  and  the  sustained  contraction  of  the  limb 
muscles  tends  to  prevent  the  blood  flowing  through  them. 
It  is  thus  forced  to  the  central  nervous  system  in  which  the 
pressure  rises,  and,  if  a  weak  spot  in  the  vessels  is -present, 
rupture  is  apt  to  occur.  But,  if  the  effort  is  still  further 
sustained,  the  high  intra-thoracic  pressure  tends  to  prevent 
proper  diastolic  filling  of  the  heart ;  blood  is  therefore  not 
sent  on  from  the  veins  into  the  arteries,  the  veins  become 
congested  and  the  arterial  pressure  falls,  less  blood  goes  to 
the  brain,  and  thus  fainting  may  result. 

In  man  the  position  of  the  abdominal  reservoir  of  blood 
at  a  lower  level  than  the  heart  increases  the  work  of  that 
,  organ.  For  this  reason,  in  people  with  a  weak  heart,  the 
sudden  assumption  of  the  erect  position  may  lead  to  a  failure 
of  the  heart  and  to  fainting.  Especially  is  this  the  case  when 
the  abdominal  wall  is  lax,  so  that  accumulation  of  blood  in 
the  abdominal  vessels  is  not  prevented.  In  the  recumbent 
position,  when  the  reservoir  is  on  the  same  level  as  the  pump, 
the  work  is  much  easier. 

On  the  other  hand,  in  the  "  head  down  position,"  the  accu- 
mulation of  blood  in  the  dependent  parts  is  prevented  in 
the  head  by  the  vessels  being  packed  inside  the  skull  and  in 
the  right  side  of  the  heart  by  the  supporting  pericardium. 

The  Time  taken  by  the  Circulation. 

This  was  first  determined  by  injecting  ferrocyanide  of 
potassium  into  the  proximal  end  of  a  cut  vein,  and  finding 
how  long  it  took  to  appear  in  the  blood  flowing  from  the 
distal  end.  From  observation  in  the  horse,  dog,  and  rabbit, 
it  appears  that  the  time  corresponds  to  about  twenty-seven 
beats  of  the  heart,  so  that  in  man  it  should  amount  to  about 
twenty-three  seconds. 

Stewart  has  investigated  the  rate  of  flow  through  different 
organs  by  injecting  salt  solution  into  the  artery,  and  by 
detecting  its  appearance  in  the  vein  by  the  change  in  the 
electric  conductivity  of  the  contents  of  the  vessel. 


SECTION  VIII 

SUPPLY  OF  NOURISHING  MATERIAL  TO  BLOOD  AND  LYMPH, 
AND  ELIMINATION  OF  WASTE  MATTER  FROM  THEM 

KESPIRATION 
I.  EXTERNAL  RESPIRATION 

IF  an  animal  be  placed  in  a  closed  chamber  filled  with 
ordinary  atmospheric  air  which  contains  by  volume  79  parts 
of  nitrogen  and  21  parts  of  oxygen,  and  if  the  air  is  examined 
after  a  time,  it  will  be  found  that  the  oxygen  has  diminished 
in  amount,  and  that  a  nearly  corresponding  amount  of  carbon 
dioxide  has  been  added. 

The  same  thing  occurs  in  aquatic  animals — the  water 
round  them  loses  oxygen  and  gains  carbon  dioxide.  An 
animal  takes  up  oxygen  and  gives  off  carbon  dioxide.  This 
is  the  process  of  external  respiration. 

I.  Respiratory  Mechanism. 

In  aquatic  animals  the  mechanism  by  which  this  process 
is  carried  on  is  a  gill  or  gills.  Each  consists  of  a  process 
covered  by  a  very  thin  layer  of  integument,  just  below  which 
is  a  tuft  of  capillary  blood  vessels.  The  oxygen  passes  from 
the  water  to  the  blood ;  the  carbon  dioxide  from  the  blood 
to  the  water. 

A  lung  is  simply  a  gill  or  mass  of  gills,  turned  outside  in 
with  air  instead  of  water  outside  the  integument.  While  in 
aquatic  gill-bearing  animals  there  is  constantly  a  fresh  supply 
of  water  passing  over  the  gills,  in  lung-bearing  animals  the 
air  in  the  lung  sacs  must  be  exchanged  by  some  mechanical 
contrivance. 

The  lungs  consist  of  myriads  of  small  thin-walled  sacs 

attached  round  the  funnel-like  expansions  in  which  the  air 

277 


278  HUMAN   PHYSIOLOGY 

passages  (infundibular  passages)  terminate.  (The  structure 
of  the  various  parts  of  the  respiratory  tract  must  be  studied 
practically.) 

Each  sac  is  lined  by  a  layer  of  simple  squamous  epithelium, 
supported  by  a  framework  of  elastic  fibrous  tissue  richly 
supplied  with  blood  vessels.  It  has  been  calculated  that,  if 
all  the  air  vesicles  in  the  lungs  of  a  man  were  spread  out  in 

one  continuous  sheet,  a  surface 
of  about  100  square  metres 
would  be  produced  and  that 
the  blood  capillaries  would 
occupy  about  75  square  metres 
of  this.  Through  these  vessels 
about  5000  litres  of  blood  would 
pass  in  twenty-four  hours. 
FIG.  130.— Scheme  of  the  Distribution  The  larger  air  passages  are 

sufported°by  pieces  of  hyaline 
cartilage  in  their  walls,  but  the 
smaller  terminal  passages,  the  bronchioles,  are  without  this 
support,  and  are  surrounded  by  a  specially  well-developed 
circular  band  of  non-striped  muscle — the  bronchial  muscle 
—which  governs  the  admission  of  air  to  the  infundibula  and 
air  sacs. 

The  lungs  are  packed  in  the  thorax  round  the  heart,  com- 
pletely filling  the  cavity. 

They  may  be  regarded  as  two  compound  elastic-walled 
sacs,  which  completely  fill  an  air-tight  box  with  movable 
walls — the  thorax — and  communicate  with  the  exterior  by 
the  windpipe  or  trachea. 

No  air  exists  between  the  lungs  and  the  sides  and  base  of 
the  thorax,  so  that  the  so-called  pleural  cavity  is  simply  a 
potential  space.  If  the  thoracic  wall  be  punctured  so  that 
this  potential  pleural  cavity  is  brought  into  connection  with 
the  air,  the  lungs  immediately  collapse  and  occupy  a  small 
space  posteriorly  round  the  large  bronchi.  This  is  due  to 
their  elasticity  (Fig.  131). 

The  lungs  are  kept  in  the  distended  condition  in  the 
thoracic  cavity  by  the  atmospheric  pressure  within  them. 

Their  elasticity  varies  according  to  whether  the  organs  are 
stretched  or  not.  As  they  collapse,  their  elastic  force  natu- 


THE   RESPIRATION 


279 


760 


rally  becomes  less  and  less ;  as  they  are  expanded,  greater  and 
greater.  Taken  in  the  average  condition  of  expansion  in 
which  they  exist  in  the  chest,  the  elasticity  of  the  excised 
lungs  of  a  man  is  capable  of  supporting  a  column  of  mercury 
of  about  30  mm.  in  height,  so  that  they  are  constantly 
tending  to  collapse  with  this  force. 

But  the  inside  of  the  lungs  freely  communicates  with  the 
atmosphere,  and  this 
at  the  sea  level  has 
a  pressure  of  about 
760  mm.  Hg.  Dur- 
ing one  part  of 
respiration  this  pres- 
sure becomes,  a  few 
mm.  smaller,  during 
another  part  a  few 
mm.  greater ;  but 
the  mean  pressure 
of  760  mm.  of  mer- 
cury is  constantly 
expanding  the  lung, 

and  acting  against  a  pressure  of  only  30  mm.  of  mercury, 
tending  to  collapse  the  lung. 

Obviously,  therefore,  the  lungs  must  be  kept  expanded 
and  in  contact  with  the  chest  wall. 

When  a  pleural  cavity  is  opened,  the  distribution  of  forces 
is  altered,  for  now  the  atmospheric  pressure  tells  also  on  the 
outside  as  well  as  on  the  inside  of  the  lung,  and  acts  along 
with  the  elasticity  of  the  organ.  So  that  now  a  force  of  760 
mm.  +  30  mm.  =  790  mm.  act  against  760  mm.,  causing  a 
collapse  of  the  lungs. 

In  the  surgery  of  the  thorax,  as  well  as  in  the  physiology 
of  respiration,  these  points  are  of  great  importance. 


FIG.  131. — Shows  the  Distribution  of  Pressure  in  the 
Thorax  with  the  chest  wall  intact,  and  with  an 
opening  into  the  Pleural  Cavity,  (j)  Indicates 
the  Atmospheric  Pressure  of  760  mm.  of  Mercury  : 
30  is  the  elasticity  of  the  Lungs  also  in  -mm.  Hg. 


II.  Physiology. 

The  process  of  respiration  consists  of  two  parts — 
1st.  The  passage  of  air  into  and  out  of  the  air  sacs. 
2nd.  The  interchange  of  gases  between  the  air  in  the  air 
vesicles  and  the  blood  in  the  capillaries. 


280  HUMAN   PHYSIOLOGY 

A.  Passage  of  air  into  and  out  of  the  Lungs. — This  is 
brought  about — 

1st.  By  the  movements  of  respiration — breathing. 

2nd.  By  diffusion  of  gases. 

The  air  is  made  to  pass  into  and  out  of  the  lungs  by 
alternate  inspiration  and  expiration. 

I.  Movements  of  Respiration — A.  Inspiration. — During  this 
act  the  thoracic  cavity  is  increased  in  all  directions — lateral, 
antero-posterior,  and  vertical.  As  the  thorax  expands,  the 
air  pressure  inside  the  lungs  keeps  them  pressed  against  the 
chest  wall,  and  the  lungs  expand  with  the  chest.  As_  a 
.result  of  this  expansion  of  the  lungs  the  pressure  inside 
becomes  less  than  the  atmospheric  pressure,  and  uir 
rushes  in  until  the  pressure  inside  and  outside  airain 
become  e<mal.  This  can  be  shown  by  placing  u  lulu,-  in 
the  mouth  or  in  a  nostril  and  connecting  it  with  a  water 
manometer. 

This  expansion  of  the  lungs  can  readily  be  deter- 
mined in  the  vertical  direction  by  percussion,  and  in 
the  horizontal  planes  by  measurement.  By  tapping 
the  chest  with  the  finger  over  the  lung  in  the  right 
intercostal  spaces,  a  resonant  note  is  produced,  while  if 
the  percussion  is  performed  below  the  level  of  the 
lung,  a  dull  note  is  heard.  If  the  lower  edge  of 
this  resonance  be  determined  before  an  inspiration, 
and  again  during  it,  it  will  be  found  to  have 
descended. 

As  a  result  of  inspiration,  the  form  of  the  chest  is 
markedly  modified,  the  change  being  best  seen  in  trans- 
verse sections.  In  expiration  the  chest  in  transverse  section 
is  an  elongated  ellipse  from  side  to  side,  in  inspiration  it 
becomes  more  circular  (Fig.  132).  The  change  from  side 
to  side  and  from  behind  forwards  is  best  marked  towards 
the  lower  part  of  the  chest,  less  marked  in  the  upper  part. 
These  changes  may  be  recorded  by  means  of  a  Cyrto- 
meter,  a  piece  of  flexible  gas  tubing  hinged  behind,  so  that 
it  can  be  modelled  to  the  chest. 

The  change  from  above  downwards  cannot  be  directly 
seen,  but  it  is  indicated  by  a  forward  movement  of  the  wall 


THE   RESPIRATION  281 

of  the  abdomen.  It  will  be  described  when  considering  the 
mechanism  by  which  it  is  brought  about. 

The  expansion  of  the  chest  in  inspiration  is  a  muscular 
act  and  is  carried  out  against  the  following  forces — 

1st.  The  elasticityjif  the  Lungs.— To  expand  the  lungs 
their  elastic  force  has  to  be  overcome,  and  the  more  they  are 
expanded  the  ^eater^^s^their  elasticity.  This  factor  there- 
fore plays  a  smaller  part  at  the~Beginmng  than  towards  the 
end  of  inspiration. 

2nd.  The  elasticity  of  the  Chest  Wall— The  resting  posi- 


FIG.  132. — Vertical-tangential,  Transverse,  and  Vertical  Mesial  Sections  of  the 
Thorax  in  Inspiration  and  Expiration. 

tion  of  the  chest  is  that  of  expiration.  To  expand  the  chest 
the  costal  cartilages  have  to  be  twisted. 

3rd.  The  elasticity  of  the  Abdominal  Wall. — As  the  cavity 
of  the  thorax  increases  downwards,  the  abdominal  viscera  are 
pushed  against  the  muscular  abdominal  wall,  which  in  virtue 
of  its  elasticity  resists  the  stretching  force. 

In  studying  how  these  changes  are  brought  about  we  may 
consider — 

1st.  Increase  in  the  thorax  frnnm.  Yy-foflf^n/y/yp^yy/ivfo. 

This  is  due  to  the  contraction  ollhe  diaphragm  (Fig.  132). 

In  expiration  this  dome-like  muscle,  rising  from  the 
vertebral  column  and  from  the  lower  costal  margin,,  arches 
upwards,  lying  for  some  distance  along  the  inner  surface  of 


282  HUMAN   PHYSIOLOGY 

the  ribs  and  then  curving  inwards  to  be  inserted  into  the 
flattened  central  tendon  to  which  is  attached  the  pericardium 
and  on  which  rests  the  heart. 

In  inspiration  the  muscular  fibres  contract.  But  the 
central  tendon  bein^  lix»id  l>y  the  pericardium  does  not 
undergo  extensive  movement.  The  result  of  the  muscular 
contraction  is  thus  to  flatten  out  the  more  marginal  part  of 
the  muscle  and  to  withdraw  it  more  or  less  from  the  chest 
wall — thus  opening  up  a  space,  the  complemental  pleura, 
into  which  the  lungs  expand. 

The  existence  of  this  complemental  pleura  is  of  great 
importance  in  cases  of  accumulation  of  fluid  in  the  chest, 
for  the  fluid  may  collect  and  push  down  the  diaphragm 
without  materially  altering  the  lower  margin  of  the  lungs. 

It  might  be  expected  that  this  contraction  of  the 
diaphragm  would  pull  inwards  the  chest  wall — but  this 
is  prevented  by  the  expansion  of  the  thorax  in  the  lateral 
and  antero-posterior  diameters  as  a  result  of  the  mechanism 
which  has  next  to  be  considered.  Nevertheless,  in  young 
children,  in  whom  the  chest  wall  is  soft,  an  indrawing  of  the 
chest  with  each  contraction  of  the  diaphragm  may  occur, 
and  may  lead  to  permanent  distortion  of  the  thorax. 

2nd.  T^wpfiAP  j^L-Jh^^JiffSt  JAL  in^Afi  g/n,tfi,r'o-fpo8t(>Tior  d'nd 
lateral  diameters. 

This  is  brought  aboqt  by  the  elevation  of  the  ribs  which 
rotate  round  the  axis  of  their  attachment  >  in  the  vertebral 
column. 

To  understand  this,  the  mode  of  the  connection  of  the  ribs 
to  the  vertebral  column  must  be  borne  in  mind.  The  head 
of  the  rib  is  attached  to  the  bodies  of  two  adjacent  vertebrae. 
The  tubercle  of  the  rib  is  attached  to  the  transverse  process 
of  the  lower  of  these  vertebrae.  From  this  the  shaft  of  the 
rib  projects  outwards,  downwards  and  forwards,  to  be  attached 
in  front  to  the  sternum  by  the  costal  cartilage.  If  the  rib 
is  made  to  rotate  round  its  two  points  of  attachment,  its 
lateral  margin  is  elevated  and  carried  outwards,  while  its 
anterior  end  is  carried  forwards  and  upwards  (see  Fig.  133). 

Further,  as  we  pass  from  above  downwards,  each  pair  of 
ribs  forms  the  arc  of  a  larger  and  larger  circle,  and  as  each 
pair  rises  it  takes  the  place  of  a  smaller  pair  above.  In  these 


THE   RESPIRATION  283 

ways  the  chest  is  increased  from  before  backwards  and  from 
side  to  side. 

The  first  pair  of  ribs  does  not  undergo  this  movement; 
the  motion  of  the  second  pair  of  ribs  is  slight,  but  the 
range  of  movement  becomes  greater  and  greater  as  we  pass 
downwards  until  the  floating  ribs  are  reached,  and  these  are 
fixed  by  the  abdominal  muscles.  This  greater  movement  is 
simply  due  to  the  greater  length  of  the  muscles  moving  them. 
The  muscles  moving  the  ribs  are  chiefly  the  external  inter- 
costal muscles,  and  these  may  be  considered  as  acting  from 
the  fixed  first  rib.  Now  if  the 
fibres  of  the  first  intercostal 
muscle  are  one  inch  in  length, 
the  second  rib  can  be  raised, 
say,  half  an  inch.  The  first  and 
second  intercostals  acting  on  the 
third  rib  will  together  be  two 
inches  in  length,  and  in  con- 
tracting they  can  raise  the  third 
rib  through,  say,  half  of  two 
inches — i.e.  one  inch.  The  first, 
second,  and  third  intercostals,  FIG.  133.— Shows  the  Movements  of 
acting  on  the  fourth  rib,  are  tho  Kibs  from  their  Position  in 

,  i  i          •       i          .-,  i  Expiration  to  their  Position  in 

three  inches  m  length,  and  can        Ration. 

therefore   raise   this  rib  half  of 

three  or  one  and  a  half  inches,  and  so  through  the  other 

ribs,  until  the  floating  ribs  fixed  by  the  abdominal  muscles 

are  reached. 

When  tho  diaphragm  takes  the  chief  part  in  inspiration 
the  breathing  is  said  to  be  abdominal  in  type — when  the 
intercostals  chiefly  act  in  raising  the  ribs  it  is  said  to  be 
thoracic.  Abdominal  breathing  is  best  marked  in  males — 
thoracic  in  females. 

Along  with  the  intercostal  muscles,  the  levatores  costarum 
also  act  in  raising  the  ribs  and  in  increasing  the  thorax  in 
the  transverse  and  antero-posterior  diameters. 

These  are  the  essential  muscles  of  inspiration,  but  other 
muscles  also  participate  in  the  act.  In  many  individuals, 
even  when  breathing  quietly,  it  will  be  seen  that  the  nostrils 


284  HUMAN  PHYSIOLOGY 

dilate  with  each  inspiration.  This  is  due  to  the  action  of 
the  dilatores  narium  which  contract  synchronously  with 
the  other  muscles  of  inspiration.  Again,  if  the  larynx  be 
examined,  it  will  be  found  that  the  vocal  cords  slightly 
diverge  from  one  another  during  inspiration.  This  is 
brought  about  by  the  action  of  the  posterior  crico-arytenoid 
muscles  (p.  308). 

Forced  Inspiration. — This  comparatively  small  group  of 
muscles  is  sufficient  to  carry  out  the  ordinary  act  of  inspira- 
tion. But,  in  certain  conditions,  inspiration  becomes  forced. 
A  forced  inspiration  may  be  made  voluntarily,  often  it  is  pro- 
duced involuntarily.  Such  forced  inspiration  is  well  seen  in 
patients  suffering  from  heart  disease,  in  whom  the  blood  is 
not  properly  oxygenated,  and  by  whom  powerful  efforts  are 
made  to  get  as  much  air  into  the  lungs  as  possible.  In  this 
condition  every  muscle  which  can  act  upon  the  thorax  to 
expand  it  is  brought  into  play.  The  body  and  spinal  column 
are  fixed  in  the  erect  position.  The  head  is  thrown  back  and 
fixed  by  the  posterior  spinal  muscles.  The  arms  and  shoulders 
are  fixed — usually  by  holding  on  to  the  sides  or  arms  of  the 
chair — and  every  muscle  which  can  act  from  the  fixed  spine, 
head  and  shoulder  girdle  upon  the  thorax  is  brought  into 
play.  Normally,  these  act  from  the  thorax  upon  the  parts 
into  which  they  are  inserted ;  now  they  act  from  their  in- 
sertion upon  their  point  of  origin.  The  sterno-mastoids, 
sterno-thyroids,  and  sterno-hyoids  assist  in  elevating  the 
thorax.  The  serratus  magnus,  pectoralis  minor,  and  upper 
fibres  of  the  pectoralis  major,  and  the  part  of  the  latissimus 
dorsi  which  passes  from  the  humerus  to  the  three  last  ribs, 
also  pull  these  structures  upwards.  The  facial  and  laryngeal 
movements  also  become  exaggerated. 

B.  In  Expiration  the  various  muscles  of  inspiration  cease 
to  act,  and  the  forces  against  which  they  contended  again 
contract  the  thorax  in  its  three  diameters. 

The  elasticity  of  the  lungs  is  no  longer  overcome  by  the 
muscles  of  inspiration,  and  the  external  atmospheric  pressure 
acting  along  with  it  drives  the  chest  wall  inwards  (see  p.  279). 

The  elasticity  of  the  costal  cartilages  and — in  the  erect 
position — the  iveight  of  the  chest  wall  cause  the  ribs  again 
to  be  depressed,  and  finally  the  elasticity  of  the  abdominal 


THE   RESPIRATION  285 

wall  drives  the  abdominal  viscera  against  the  relaxed  dia- 
phragm and  again  arches  it  towards  the  thorax,  squeezing  its 
marginal  portion  against  the  ribs  and  occluding  the  comple- 
mental  pleura.  Experimental  evidence  shows  that  the  inter- 
nal intercostals  contract  with  each  expiration,  and  help  to 
draw  the  ribs  downwards. 

Ordinary  expiration  is  thus  normally  mainly  a  passive 
act,  being  simply  a  return  of  the  thorax  to  the  position  of 
rest.  But  voluntarily,  and  in  certain  conditions  involuntarily, 
expiration  may  be  forced.  Forced  expiration  is  then  partly 
due  to  the  above  factors,  and  partly  due  to  the  action  of 
muscles.  Every  muscle  which  can  in  any  way  diminish  the 
size  of  the  thorax  comes  into  play. 

Chief  of  these  are  the  abdominal  muscles,  which  by 
compressing  the  viscera  push  them  upwards  and  press 
the  diaphragm  further  up  into  the  thorax.  At  the  same 
time  by  acting  from  the  pelvis  to  pull  down  the  ribs 
they  decrease  the  thorax  from  side  to  side,  and  from  before 
backwards. 

The  serratus  posticus  inferior  and  part  of  the  sacro-lumbalis 
pull  downwards  the  lower  ribs,  and  the  triangularis  sterni  also 
assists  in  this. 

By  this  constriction  of  the  thorax,  brought  about  by 
ordinary  or  by  forced  expiration,  the  air  inside  is  compressed 
and  the  pressure  raised.  During  ordinary  expiration  the 
highest  pressure  reached  is  about  2  to  3  mm.  Hg,  in  forced 
expiration  about  80  mm. 

The  pressure  of  the  air  outside  is  less  than  this,  and  the  air 
inside  the  chest  is  driven  out. 

Special  Respiratory  Movements.  --  There  are  several 
peculiar  and  special  reflex  actions  of  the  respiratory  muscles, 
each  caused  by  the  stimulation  of  a  special  district,  and 
each  having  a  special  purpose. 

Coughing. — This  consists  of  an  inspiration  followed  by  a 
strong  expiratory  effort  during  which  the  glottis  is  constricted 
but  is  forced  open  repeatedly  by  the  current  of  expired  air. 
It  is  generally  due  to  irritation  of  the  respiratory  tract,  and 
its  object  is  to  expel  foreign  matters. 

Sneezing. — This  is  generally  produced  by  irritation  of  the 
nasal  mucous  membrane.  It  consists  in  an  inspiratory  act 


286 


HUMAN  PHYSIOLOGY 


followed  by  a  forced  expiration  during  which,  by  contraction 
oflhe  pillars  of  the  fauces  and  descent  of  the  soft  palate,  the 
air  is  forced  through  the  nose. 

Hiccough  consists  in  a  sudden  reflex  contraction  of  the 
diaphragm  causing  a  sudden  inspiration  which  is  interrupted 
by  a  spasmodic  contraction  of  the  glottis.  Abdominal  irrita- 
tion is  its  chief  cause. 

Sighing  and  Yawning  are  deep  involuntary  inspirations 
which  serve  to  accelerate  the  circulation  of  the  blood  when 
from  any  cause  this  becomes  less  active  (see  p.  294).  They  are 
probably  due  to  cerebral  anaemia,  which  they  help  to  correct 
by  increasing  the  general  arterial  pressure. 

II.  Amount  of  air  respired. — The  amount  of  air  respired 
varies  according  to  whether  the  respirations  are  ordinary  or 
forced. 

In  ordinary  respiration  about  300  ccms.  of  air  enter  and 
leave  the  chest.     This  is  called  the  tidal  air.     Its  amount 
varies  with  the  size  and  muscular  develop- 
ment of  the  chest. 

By  a  forced  inspiration  a  much  larger 
Co'^c.f-     (luant^ty  °f  ftir  mav  be  made  to  pass  into 


/5OO     CC 


the  lungs  —  a  quantity  varying  with  the 
size  and  strength  of  the  individual  —  but 
on  an  average  about  1500  ccms. 
This  is  called  the  complemental  air. 
By  forced  expiration  an  amount  of  air 
FIG.  134.—  The  amount    much  larger  than  the  tidal  can  be  ex- 
of    air    respired    in    pelled,   an    amount    usually    about    the 
same  as  the  complemental  air,  and  called 

the  reserve  air. 

The  total  amount  of  air  which  an  in- 

dividual can  draw  into  and  drive  out  of  his  lungs  is  a  fair 
measure  of  the  size  and  muscular  development  of  the  thorax, 
and  it  has  been  called  the  vital  capacity  of  the  thorax. 
This  vital  capacity  may  be  measured  by  means  of  a  spiro- 
meter.  Its  amount,  as  thus  indicated,  depends  a  good  deal 
upon  practice  ;  the  instrument,  therefore,  cannot  be  con- 
sidered as  of  much  practical  value,  and  a  more  reliable 
conclusion  as  to  the  vital  capacity  may  be  arrived  at  by 


tion  and  expiration. 


THE   RESPIRATION  287 

measuring  the  circumference  of  the  chest  in  expiration  and 
in  inspiration. 

Even  after  the  whole  of  the  reserve  air  has  been  driven 
out  of  the  chest,  a  considerable  quantity  still  remains  in 
the  air  vesicles,  its  amount  depending  upon  the  size  of  the 
chest,  but  averaging  about  2000  ccms.  This  is  called  the 
residual  air. 

This  very  important  point  must  always  be  remembered, 
that  the  air  taken  into  the  chest  never  fills  the  air  vesicles, 
and  that  air  is  never  driven  completely  out  of  them.  The 
air  in  them  is  thus  not  changed  by  the  movements  of  respira- 
tion but  by  the  process  of  diffusion. 

III.  Interchange  of  air  in  the  lungs  by  diffusion  of  gases.— 

It  has  been  shown  that  only  the  air  in  the  trachea  and  bronchi 
undergoes  exchange  in  mass,  but  that  the  air  of  the  vesicles  is 
not  driven  out  of  the  chest.  The  renewal  of  co^  o 
this  air  depends  upon  the  diffusion  of  gases. 

If  two  gases  are  brought  into  relationship 
with  one  another  they  diffuse  and  tend  to 
form  a  mixture  uniform  throughout.  But, 
if  at  one  point  of  a  system,  one  gas  is  con- 
stantly being  taken  away  and  another  con- 
stantly added,  there  will  be  a  constant 
diffusion  of  the  former  towards  the  part 
where  it  is  being  taken  up,  and  a  constant 
diffusion  of  the  latter  away  from  the  point 
from  which  it  is  being  given  off.  Suppose  a  FIG.  135.  —  Shows 
tube  (Fig.  135)  containing  oxygen  and  carbon  *he  diffusion  of 

j-       -j  j  ^tA      /  J      £  ^          Oxygen  into,  and 

dioxide,  and  suppose  that  at  one  end  of  the  of  carbon  Dioxide 
tube  oxygen  is  constantly  being  taken  up  and  out  of,  the  Air 
carbon  dioxide  constantly  given  off,  a  diffu-  Ves 
sion  of  the  gases  in  the  direction  indicated  by  the  arrows 
will  continually  go  on,  and  thus  a  constant  supply  of  oxygen 
will  be  conveyed  to  the  bottom  of  the  tube,  while  carbonic 
acid  will  constantly  be  cleared  out.  This  is  exactly  the  con- 
dition in  the  lungs. 

The  lower  part  of  the  tube  corresponds  to  the  air  vesicles 
—the  upper  part  to  the  air  passage  in  which  the  air  is  con- 
stantly being  exchanged  by  the  movement  of  respiration. 


t 


288  HUMAN   PHYSIOLOGY 

The  mechanism  by  which  the  gaseous  exchange  in  the 
alveoli  is  carried  out  is  thus  a  double  one. 

IY.  Breath  Sounds. — The  air  as  it  passes  into  and  out  of 
the  lungs  produces  sounds,  which  may  be  heard  on  listening 
over  the  thorax.  The  character  of  the  breath  sounds  are  of 
the  utmost  importance  in  the  diagnosis  of  diseases  of  the 
lungs,  and  must  be  studied  practically. 

On  listening  over  the  trachea  or  over  the  bifurcation  of 
the  bronchi  behind  (between  the  4th  and  5th  dorsal  vertebra?), 
a  harsh  sound,  something  like  the  guttural  ch  (German  ich), 
may  be  heard  with  inspiration  and  expiration.  This  is  called 
the  bronchial  sound. 

If  the  ear  be  applied  over  a  spot  under  which  a  mass  of 
air  vesicles  lies,  a  soft  sound,  somewhat  resembling  the  sound 
of  gentle  wind  among  leaves,  may  be  heard  throughout 
inspiration  and  for  a  third  or  less  of  expiration.  This  is 
called  the  vesicular  sound. 

When  the  air  vesicles  become  consolidated  by  disease  the 
vesicular  sound  is  lost  and  the  bronchial  sound  takes  its 
place.  The  cause  of  the  vesicular  character  is  therefore  to 
be  sought  in  the  vesicles,  infundibula,  or  small  bronchi. 

The  cause  of  the  bronchial  sound  has  been  determined  by 
experiments  on  horses.  It  has  already  been  seen  that  a 
column  of  fluid — and  the  same  is  true  of  a  column  of  air — 
moving  along  a  tube  of  uniform  calibre,  or  with  the  calibre 
only  slowly  changing,  produces  no  sound.  Any  sudden 
alteration  in  calibre  produces  vibration  and  a  musical  sound, 
as  explained  on  p.  233.  The  first  sharp  constriction  of  the 
respiratory  tract  is  at  the  glottis,  and  it  is  here  that  the 
bronchial  sound  is  produced.  If  the  trachea  is  cut  below 
the  larynx  and  drawn  freely  outwards,  the  bronchial  sound 
at  once  stops,  and  the  vesicular  sound  becomes  lower  and  less 
distinct. 

The  cause  of  the  vesicular  sound  is  not  so  satisfactorily 
explained.  It  is  in  part  due  to  propagation  of  the  bronchial 
sound,  altered  by  passing  through  vesicular  tissue ;  but  it  is 
also  probably  due  to  the  expansion  and  contraction  of  the 
air  vesicles  drawing  in  and  expelling  air,  either  through  their 
somewhat  narrow  openings  into  the  infundibula,  or  through 


THE  RESPIRATION  289 

the  narrow  opening  of  the  infundibula  into  the  bronchioles. 
The  reason  why  the  sound  is  best  heard  during  inspiration 
may  be  that  the  sound  is  best  conducted  in  the  direction  of 
the  air  stream. 

Y.  Rhythm  of  Respiration. — These  movements  of  respira- 
tion are  carried  on  in  a  regular  rhythmic  manner.  Their 
rate  varies  with  many  factors ;  but  the  average  number  of 
respirations  per  minute  in  the  adult  male  is  about  sixteen, 
or  about  one  to  every  four  or  five  beats  of  the  heart.  The 
rate  of  respirations  may  be  modified  by  the  will,  and, 
therefore,  in  counting  the  respirations  in  a  patient,  it  is  well 
to  prevent  his  being  aware  of  what  is  being  done.  Similarly, 
on  account  of  this  influence  of  the  upper  part  of  the  brain, 
the  respirations  should  not  be  counted  while  the  patient  is 
excited  or  nervous. 

The  most  important  factor  modifying  the  rate  of  respira- 
tion is  the  age  of  the  individual.  The  following  table  gives 
the  average  rate  at  different  ages : — 

Under    1  year  .         .  44 

„         5  years          .         .  26 

„       20      „  .  19 

.  Adult         ....  16 

The  other  modifications  in  the  rate  of  breathing  will  be 
better  understood  after  studying  the  nervous  mechanism  of 
respiration. 

In  these  respiratory  movements  the  phase  of  inspiration 
bears  a  certain  proportion  to  that  of  expiration.  Inspiration 
is  much  more  rapid  than  expiration  (see  Fig.  134).  As  soon 
as  inspiratian  is  completed,  a  reverse  movement  occurs,  which 
is  at  first  rapid,  but  gradually  becomes  slower,  and  may  be 
followed  by  a  pause,  during  which  the  chest  remains  in  the 
collapsed  condition.  The  existence  and  duration  of  this 
pause  varies  much,  and  it  may  really  be  considered  as  the 
terminal  period  of  expiration.  Considering  it  in  this  light, 
we  may  say  that  inspiration  is  to  expiration  as  6  is  to  7. 

YI.  Nervous  Mechanism  of  Respiration. — The  rhythmic 
movements  of  respiration  require  the  harmonious  action  of 

19 


290 


HUMAN  PHYSIOLOGY 


a  number  of  muscles,  and  this  is  directed  by  the  nervous 
system. 

The  diaphragm  is  supplied  by  the  phrenic  nerves  rising 
from  the  third  and  fourth,  and  partly  from  the  fifth  cervical 
nerves.  The  intercostals  are  supplied  by  branches  from  their 
corresponding  dorsal  nerves. 

If  the  spinal  cord  be  cut  below  the  fifth  cervical  nerve  the 
intercostal  muscles  cease  to  act.  If  the  section  is  made  above 
the  third  cervical  nerve,  the  diaphragm,  too,  is  paralysed, 
and  the  animal  dies  of  suffocation. 

Respiratory  Centre. — Obviously,  then,  there  is  some  ner- 


R.C. 


Diaph 


FIG.  130.— Nervous  Mechanism  of  Respiration.  R.C.,  Respiratory  Centre;  Cut., 
Cutaneous  Nerves;  Ph.,  Phrenics;  In.  C.,  Intercostal  Nerves;  P.,  Pul- 
monary Branches  of  Vagus;  S.L.,  Superior  Laryngeal  Branch  of  Vagus; 
I/a.,  the  Larynx;  O.  Ph.,  Glossopharyngeal  Nerve;  Diaph.,  Diaphragm. 

vous  mechanism  above  the  spinal  cord  presiding  over  these 
muscles. 

Removal  of  the  brain  above  the  medulla  oblongata  does 
not  stop  the  respiratory  rhythm. 

The  mechanism  must,  therefore,  be  situated  in  the  medulla 
oblongata. 

If  the  medulla  is  split  into  two  by  an  incision  down  the 
middle  line,  respiration  continues,  but  the  two  sides  do  not 
always  act  at  the  same  rate.  The  mechanism,  then,  is  bi- 
lateral, but  normally  the  two  parts  are  connected,  and  thus 
act  together. 

Destruction  of  the  part  of  the  medulla  lying  near  the  root 
of  the  vagus  arrests,  respiration,  and  it  may,  therefore,  be 


THE  RESPIRATION  291 

concluded  that  the  nervous  mechanism  presiding  over  this 
act  is  situated  there. 

It  must  not  be  imagined  that  this  centre  sends  fibres 
directly  to  the  muscles  concerned  in  respiration.  The 
nerves  passing  to  these  come  from  the  cells  in  the  grey 
matter  of  the  spinal  cord,  and  it  is  by  influencing  the 
activity  of  these  cells  that  the  respiratory  centre  controls 
the  act  of  respiration. 

Since  expiration,  when  forced,  is  a  complex  muscular  act, 
it  is  reasonable  to  suppose  that  the  respiratory  centre  con- 
tains two  parts — one  presiding  over  inspiration,  one  presiding 
over  expiration.  While  the  inspiratory  centre  is  constantly 
in  rhythmic  action,  the  expiratory  centre  is  only  occasionally 
at  work. 

Mode  of  Action  of  the  Respiratory  Centre. — Both  parts  of 
the  respiratory  centre  are  under  the  control  of  higher  nerve 
centres,  and  through  these  they  may  be  thrown  into  action 
at  any  time,  or  even  prevented  from  acting  for  the  space  of 
a  minute  or  so.  But,  after  the  lapse  of  this  period,  the 
respiratory  mechanism  proceeds  to  act  in  spite  of  the  most 
powerful  attempts  to  prevent  it. 

To  determine  its  mode  of  action  the  influence  of  afferent 
nerves  upon  the  centre  must  be  considered. 

Yagus. — Since  the  vagus  is  the  nerve  of  the  respiratory 
tract  we  should  expect  it  to  have  an  important  influence  on 
the  centre  (Fig.  136). 

Section  of  one  vagus  causes  the  respiration  to  become 
slower  and  deeper ;  but,  after  a  time,  the  effect  wears 
off,  and  the  previous  rate  and  depth  of  respiration  is 
regained  (Fig.  137). 

Section  of  both  vagi  causes  a  very  marked  slowing  and 
deepening  of  the  respiration,  which  persists  for  some  time, 
and  passes  oft  slowly  and  incompletely.  Now,  if  after  the 
vagi  have  been  cut,  the  connection  of  the  centre  with  the 
upper  brain  tracts  is  severed,  the  mode  of  action  of  the 
centre  totally  changes.  Instead  of  discharging  rhythmically 
it  remains  for  a  long  period  at  rest,  then  the  inspiratory 
centre  discharges  violently,  causing  a  strong  and  prolonged 
contraction  of  the  muscles  of  inspiration.  This  passes  off, 
and  again  a  period  of  rest  of  variable  duration  sets  in,  to  be 


292  HUMAN  PHYSIOLOGY 

again  interrupted  by  another  more  or  less  long  and  strong 
discharge. 

Separation  of  the  respiratory  centre  from  the  vagi  and 
upper  brain  tracts  brings  about  a  loss  of  its  rhythmic 
action,  but  does  not  stop  its  activity.  The  centre  owes  the 
rhythmic  nature  of  its  action  to  afferent  impulses.  These 
afferent  impulses  reach  it  normally  through  the  vagi,  but 
when  these  are  cut  the  upper  brain  takes  upon  itself  the 
function  of  maintaining  the  rhythm. 

To  investigate  further  this  influence  of  the  vagus  it  is 
necessary  to  study  the  effect  of  stimulating  the  nerve. 

Strong  stimulation   of   the   pulmonary  branches   of  one 


d' 


FlO.  137. — Tracings  of  the  Respirations — Downstrokes,  Inspiration;  Upstrokes, 
Expiration.  At  a  one  Vagus  Nerve  was  cut ;  at  6  the  second  was  divided  ; 
at  c  the  Upper  Brain  Tracts  also  were  cut  off ;  d  and  d'  show  the  effect  of 
Stimulating  the  Glossopharyngeal  Nerve. 

vagus  (vagus  below  the  origin  of  the  superior  laryngeal) 
causes  the  respiration  to  become  more  and  more  rapid,  the 
inspiratory  phase  being  chiefly  accentuated.  If  the  stimulus 
is  very  strong  respirations  are  stopped  in  the  phase  of  in- 
spiration. Weak  stimuli,  on  the  other  hand,  may  cause 
inhibition  of  inspiration. 

Such  experiments  prove  that  impulses  are  constantly 
travelling  from  the  lungs  to  the  centre  whereby  the  rhythmic 
activity  of  the  centre  is  maintained. 

How  do  these  impulses  originate  in  the  lungs  ?  Appa- 
rently from  their  alternate  expansion  and  contraction. 

If  the  lungs  be  forcibly  inflated — e.g.  with  a  bellows — the 
inspiration  becomes  feebler  and  feebler  and  finally  stops. 


THE  RESPIRATION  293 

The  nature  of  the  gas,  if  non-irritant,  with  which  this  in- 
flation is  carried  out  is  of  no  consequence.  If  on  the  other 
hand  the  lungs  be  collapsed  by  sucking  air  out  of  them,  the 
inspiration  becomes  more  and  more  powerful,  and  may  end 
in  a  spasm  of  the  inspiratory  muscles. 

This  shows  that  with  each  expiration  a  stimulus  passes  up 
the  vagus  which  acts  upon  the  inspiratory  centre  to  make  it 
discharge.  The  vagus  is  thus  a  true  excito-motor  nerve, 
making  the  centre  act  in  a  reflex  manner.  With  each  col- 
lapse of  the  lung  the  vagus  is  thrown  into  action,  as  the 
lungs  expand  it  ceases  to  act,  and,  as  a  result,  the  inspiratory 
centre  stops  acting,  the  muscles  of  inspiration  cease  to  con- 
tract, and  expiration  occurs. 

While  ordinary  respiration  may  thus  be  considered  as  a 
rhythmic  reflex  act,  it  must  not  be  forgotten  that  the  respira- 
tory centre  can  and  does  act  rhythmically  under  the  influence 
of  the  higher  centre,  or  a-rhythmically  and  spasmodically 
when  these  as  well  as  the  vagi  are  severed  from  it. 

So  far  the  pulmonary  branches  of  the  vagus  alone  have 
been  considered. 

But  the  upper  part  of  the  respiratory  tract,  the  larynx, 
receives  its  sensory  fibres  from  this  nerve.  Section  of  the 
superior  laryngeal  branch  of  the  vagus  does  not  alter  the 
rhythm  of  respiration.  Stimulation  of  the  upper  end  of  the 
cut  nerve  causes  first  an  inhibition  of  inspiration  and,  if 
stronger,  produces  forced  expiratory  acts.  This  is  well  illus- 
trated by  the  very  common  experience  of  the  effect  of  a 
foreign  body,  such  as  a  crumb,  in  the  larynx.  The  fit  of 
coughing  is  a  series  of  expiratory  acts  produced  through 
this  nerve. 

Another  set  of  visceral  nerves  having  an  important  influ- 
ence on  the  respiratory  rhythm  are  the  splanchnics. 

When  these  are  stimulated  inspiration  is  inhibited.  Every 
one  has  experienced  the  "loss  of  wind"  as  the  result  of  a 
blow  on  the  abdomen. 

The  glossopharyngeal,  which  supplies  the  back  of  the 
tongue,  when  stimulated,  as  by  the  passage  of  food  in  the 
act  of  swallowing,  causes  an  instant  arrest  of  the  respiratory 
movements  either  in  inspiration  or  expiration.  The  advan- 
tage of  this  in  preventing  the  food  as  it  is  swallowed  from 


294  HUMAN  PHYSIOLOGY 

passing  into  the  trachea  must  be  obvious  (Fig.  137,  d 
and  d'). 

Stimulation  of  the  Cutaneous  Nerves  stimulates  the  in- 
spiratory  centre  and  causes  a  deep  inspiration.  This  is  seen 
when  cold  water  is  dashed  upon  the  skin,  and  is  more  clearly 
demonstrated  in  animals  with  the  vagi  and  upper  nerve 
tracts  cut  across.  In  such  animals  if  the  skin  be  touched  an 
inspiratory  movement  is  made. 

The  action  of  the  respiratory  centre  is  thus  regulated  by 
many  afferent  nerves.  Its  activity  is  also  modified  by  the 
condition  of  the  blood  and  lymph  going  to  it. 

If  the  supply  of  blood  to  the  medulla  be  interfered  with — 
e.g.  by  ligaturing  the  arteries  to  the  head — breathing  becomes 
deeper  and  more  laboured.  The  activity  of  the  respiratory 
centre  is  increased  in  the  same  way  if  the  proper  exchange 
of  gases  between  the  blood  and  the  air  in  the  lungs  is  inter- 
fered with.  This  may  be  due  either  to  the  withdrawal  of 
something  necessary  for  the  nutrition  of  the  nerve  cells  of 
the  centre,  or  to  the  accumulation  of  products  of  activity. 

One  of  the  most  important  substances  yielded  by  the 
blood  for  the  nutrition  of  cells  is  oxygen.  That  the  absence 
of  oxygen  is  alone  sufficient  to  stimulate  the  activity  of  the 
respiratory  centre  is  shown  by  the  fact  that  it'  an  animal  is 
made  to  breathe  an  atmosphere  of  nitrogen  which  prevents 
fHel)Iood  from  obtaining  oxygen,  but  docs  not  interfere  with 
its  getting  rid  of  its  carbon  dioxide,  increase  in  the  activity  of 
the  respiratory  centres  is  produced. 

This  experiment  does  not,  however,  exclude  the  possibility 
that  the  accumulation  of  carbon  dioxide  may  also  stimulate 
the  centre.  If  an  animal  be  supplied  with  air  in  which  there 
is  an  abundance  of  oxygen,  but  which  is  loaded  with  carbonic 
acid,  the  result  is  that  the  carbonic  acid  of  the  blood  is  not 
got  rid  of.  In  such  an  animal  the  breathing  becomes  quicker, 
deeper  and  more  laboured ;  but  ultimately  there  is  a  tendency 
for  the  breathing  to  diminish  and  for  the  animal  to  sink  into 
a  state  of  deep  coma.  Since,  under  ordinary  conditions,  the 
amount  of  oxygen  in  the  blood  does  not  materially  vary, 
while  the  amount  of  C02  may  be  considerably  altered,  it  is 
probable  that  the  increase  of  C02  under  certain  conditions  is 
the  more  frequent  stimulus  to  the  respiratory  centre. 


THE   RESPIRATION  295 

The  accumulation  of  other  Waste  Products  in  the  blood 

undoubtedly  stimulates  these  nerve  cells.  Muscular  exertion 
increases  the  activity  of  respiration.  That  this  is  really  due 
to  the  accumulation  of  products  of  muscle  waste  in  the  blood 
and  not  to  any  reflex  influence  through  the  nerves  is  shown 
by  the  fact  that  it  occurs  in  a  dog  when  the  spinal  cord  is 
cut  across,  and  the  muscles  of  the  hind  limb  are  made  to 
contract  by  stimulating  the  nerves  with  electricity.  The  fact 
that  the  injection  of  acid  into  the  blood  of  animals,  which 
are  unable  rapidly  to  neutralise  it,  increases  the  respirations, 
suggests  the  possibility  that  sarcolactic  acid  may  be  among 
the  substances  which  can  stimulate  these  nerve  cells. 

Yet  another  factor  of  importance  in  modifying  the  activity 
of  this  centre  is  the  temperature  of  the  animal.  Increase  in 
temperature  accelerates  the  rate  of  the  heart — so,  too,  it 
accelerates  the  rate  of  the  respirations,  and  in  about  the 
same  proportion,  as  is  seen  in  feverish  attacks,  where  pulse 
and  respiration  are  proportionately  quickened  so  that  their 
ratio  remains  unaltered.  When  the  respiratory  rate  rises 
out  of  proportion  to  the  rate  of  the  pulse,  it  is  usually  an 
indication  that  some  pulmonary  irritation  is  present. 

The  lungs  and  heart  being  packed  tightly  together  in  the 
air-tight  thorax,  and  both  undergoing  periodic  changes, 
necessarily  influence  one  another.  At  the  same  time,  the 
close  proximity  of  the  respiratory  and  cardio-motor  centres 
in  the  medulla  seems  to  lead  to  the  activity  of  one  influencing 
the  other. 

Influence  of  Respiration  on  Circulation. — The  circulation 
is  modified  in  two  ways  by  respiration.  First,  the  rate  of 
the  heart ;  and  second,  the  arterial  blood  pressure  undergo 
alterations. 

1st.  Rate  of  Heart. — If  a  sphygmographic  trace  giving  the 
pulse  waves  during  the  course  of  two  or  three  respirations  be 
examined,  it  will  be  found  that  during  inspiration  the  heart 
is  acting  more  rapidly,  while  during  expiration  its  action  is 
slowed. 

If  the  vagus  is  cut  these  changes  are  not  seen,  showing 
that  the  inspiratory  acceleration  is  not  the  result  simply  of 
the  larger  amount  of  blood  which  enters  the  heart  during 


296  HUMAN  PHYSIOLOGY 

inspiration,  but  is  really  due  to  changes  in  the  cardio-motor 
centre — the  accelerating  part  of  which  has  its  activity  in- 
creased during  inspiration,  while  the  inhibitory  part  is  more 
active  during  expiration.  This  is  thus  a  reflex  effect  from 
the  lung  through  the  vagus,  and  it  may  be  in  part  due  to  the 
proximity  of  the  centres  in  the  medulla. 

But  not  only  is  the  pulse  more  rapid  during  inspiration 
and  slower  during  expiration,  but  the  waves  are  smaller 
during  inspiration  and  larger  during  expiration.  The  size 
of  the  wave  depends  much  upon  the  pressure  of  blood  in  the 
arteries,  and  this  change  in  the  pulse  thus  leads  to  the  fuller 
consideration  of  the  changes  in  the  arterial  pressure  due  to 
respiration. 

2nd.  Changes  in  Blood  Pressure. — If  a  tracing  of  the 
arterial  pressure  and  of  the  respiratory  movements  are  taken 
at  the  same  time,  it  is  found  that  there  is  a  general  rise  of 
pressure  during  inspiration  and  a  general  fall  during  expira- 
tion, but  that  at  the  beginning  of  inspiration  the  pressure  is 
still  falling,  and  at  the  beginning  of  expiration  it  is  still 
rising  (Fig.  120,  p.  256).  This  influence  of  respiration  on 
arterial  pressure  is  chiefly  a  mechanical  one,  depending  on 
the  variations  in  the  pressure  in  the  thorax  during  inspiration 
and  expiration. 

During  inspiration  the  pressure  in  the  thorax  falls  to 
below  the  atmospheric  pressure,  and  thus  during  this  period 
the  heart  and  great  vessels  are  under  a  diminished  pressure. 
This  diminution  in  pressure  has  little  influence  on  the  thick- 
walled  ventricles  and  arteries,  but  tells  markedly  on  the  thin- 
walled  auricles  and  veins.  In  these  there  occurs  a  diminution 
in  pressure,  which,  in  the  case  of  the  vena  cava,  may  fall 
below  an  atmospheric  pressure,  and  as  a  result  an  increased 
flow  of  blood  into  these  vessels  from  the  veins  outside  the 
thorax  takes  place  (Fig.  129,  p.  275). 

But  when  more  blood  enters  the  heart  the  activity  of  the 
organ  is  increased,  and  more  blood  is  pumped  through  it  into 
the  arteries,  and  the  pressure  in  these  rises.  This  explains 
the  great  rise  in  arterial  pressure  during  inspiration. 

During  expiration  the  pressure  in  the  thorax  rises  to  above 
the  atmospheric  pressure,  and  thus  the  pressure  on  the  vessels 
in  the  thorax  is  increased.  This  tells  on  the  thin-walled 


THE  RESPIRATION  297 

veins  and  auricles,  and  thus  the  flow  of  blood  into  them  is 
retarded  (Fig.  129),  and,  less  blood  passing  into  the  heart,  less 
is  pumped  into  the  arteries,  and  the  arterial  pressure  falls. 

This,  however,  does  not  explain  the  slight  fall  of  pressure 
at  the  beginning  of  inspiration,  or  the  slight  rise  at  the 
beginning  of  expiration.  To  understand  these,  the  action  of 
the  pulmonary  circulation  has  to  be  taken  into  account. 

As  inspiration  develops  the  lungs  are  dilated,  and  the 
capillaries  in  them  are  also  expanded.  These  expanding 
capillaries  require  more  blood  to  fill  them.  They  are  situated 
on  the  course  of  the  blood  from  the  right  side  to  the  left  side 
of  the  heart,  and  thus  blood  is  retained  from  this  stream  to 
fill  them,  and  less  blood  passes  on  into  the  left  side  of  the 
heart  and  out  into  the  arteries,  and  thus  at  the  beginning 
of  inspiration  a  small  fall  in  the  arterial  pressure  occurs 
(Fig.  129). 

Similarly,  at  the  beginning  of  expiration  the  lungs  are 
compressed  and  their  blood  vessels  squeezed,  and  thus  the 
blood  is  driven  out  from  them.  Now,  this  blood  cannot  pass 
back  into  the  right  side  of  the  heart,  so  it  must  pass  on  into 
the  left  side — more  blood  is  driven  into  the  arteries,  and  thus 
the  pressure  rises.  As  soon,  however,  as  the  excess  of  blood 
has  been  squeezed  out  of  the  lungs,  the  contracted  state  of 
the  vessels  further  retards  the  passage  of  blood  to  the  left 
side  of  the  heart,  and  assists  in  diminishing  the  arterial 
pressure. 

Influence  of  the  Action  of  the  Heart  on  Respiration. 

—The  heart  lies  in  the  thorax  surrounded  by  the  elastic 
lungs.  As  it  contracts  and  dilates  it  must  alternately  pull 
upon  and  compress  the  lungs,  and  thus  tend  to  cause  an  inrush 
and  an  outrush  of  air — the  cardio-pneumatic  movement. 

If  a  simultaneous  tracing  of  the  heart-beat  and  of  the 
movements  of  the  air  column  be  taken,  it  will  be  seen  that 
at  the  beginning  of  ventricular  systole  there  is  a  slight  out- 
rush  of  air  from  the  lungs,  probably  caused  by  the  blow  given 
to  the  lungs  by  the  suddenness  of  the  systolic  movement. 
This  is  followed  by  a  marked  inrush  of  air  corresponding  to 
the  outflow  of  blood  from  the  ventricles,  and  caused  by  the 
fact  that  the  contracting  ventricles  draw  on  and  expand  the 


298 


HUMAN  PHYSIOLOGY 


lungs.  This  is  followed  by  a  slower  outrush  of  air  corre- 
sponding to  the  active  filling  of  the  ventricles  during  the 
beginning  of  ventricular  diastole.  Lastly,  during  the  period 

of  passive  diastole,  the  cardio- 
pneumatic  movements  of  air  are 
in  abeyance. 

These  cardio-pneumatic  move- 
ments are  of  great  importance  in 
animals  which  hibernate.  Dur- 
ing their  winter  sleep  the  ordi- 
nary respirations  almost  stop, 
but  a  sufficient  gaseous  inter- 
change is  kept  up  by  these 
cardio-pneumatic  movements. 

FIG.  138,-To  show  relations  of        In    the    examination    of    the 
Cardio-pneumatic  Movements,    heart  sounds  they  must  always 

A  to  Cardiac  Cycle,  B.     In  A     be     bome     ^     mind>    because;    if 

there  is  any  constriction  in  a 
small  bronchus  near  the  heart, 
the  rush  of  air  through  this  may  give  rise  to  a  murmuring 
sound,  in  character  very  like  a  cardiac  murmur  and  syn- 
chronous with  the  heart's  action.  On  making  the  patient 
cough,  such  a  murmur  at  once  disappears. 


the  upstrokes  are  Expiratory, 
the  downstrokes  Inspiratory. 


B.  Interchange  between  the  Air  breathed  and  the   Blood 
in  the  Lung  Capillaries. 

I.  Effect    of   Respiration   upon    the    Air    breathed. — To 

determine  this,  some  method  of  analysing  the  air  exhaled 
must  be  employed.  In  the  case  of  larger  animals  a  mask 
with  a  valve  to  allow  the  collection  of  the  expired  air  may  be 
used,  while  small  animals  may  be  placed  in  a  closed  space  to 
which  measured  quantities  of  air  are  admitted,  and  samples 
of  the  air  drawn  off,  or  the  whole  air  drawn  off,  may  be 
collected  and  analysed. 

The  following  table  shows  the  average  percentage  coin- 
pos^tion  of  the  air  inspired  and  the  air  expired : — 

Per  Cent,  of  N.  O.  CO2. 

Inspired  air    .        .        .79  21  0 

Expired  air    .         .         .       79  17  4 


THE  KESPIRATION 


299 


i.e.  about  4  per  cent,  of  oxygen  is  taken  from  the  air,  and 
about  4  per  cent,  of  carbon  dioxide  is  added  to  it.  In  man 
the  amount  of  carbon  dioxide  given  off  is  smaller  than  the 
amount  of  oxygen  taken  up.  The  proportion  between  the 

"Q  +  k is  called  the  Respiratory  Quotient,  and  it  is 

thus  less  than  unity — usually   about   '8   to  *9 — that  is,  for 

every  five  volumes  of  oxygen  taken  up  only  four  volumes  are 

given  off  in  carbon  dioxide,  the 

remainder   being   combined  with 

hydrogen    to    form    water.     The 

various    factors    modifying    this 

quotient  will  be  considered  while 

dealing  with   the   extent  of  the 

respiratory  changes. 


FIG.  139.— Shows  the  Composition 
of  Inspired  and  of  Expired  Air. 


Expired  air  usually  contains  more  water  than  inspired  air. 
It  is  always  saturated  with  watery  vapour. 

Expired  air  also  contains  small  amounts  of  organic  matter, 
which  give  it  its  offensive  odour.  These  may  possibly  be  to 
a  small  extent  formed  in  the  lungs,  but  are  to  a  greater 
extent  produced  by  putrefactive  change  in  the  mouth  and 
nose.  It  is  probable  that  the  accumulation  of  these  products 
in  the  air  is  one  of  the  great  causes  of  the  injurious  effect  of 
the  "  foul  air  "  in  overcrowded  spaces. 

Expired  air  is  usually  warmer  than  inspired  air,  because 
usually  the  body  is  warmer  than  the  surrounding  atmosphere. 
When,  however,  the  temperature  of  the  air  is  higher  than 
that  of  the  body,  the  expired  air  is  cooler  than  the  inspired. 

This  is  illustrated  by  the  figures  of  an  experiment — 


Temperature  of  Inspired  Air. 

6-3°  C. 

17-19°  C. 

41°  C. 

44°  C. 


Temperature  of  Expired  Air. 

30°  C. 
37°  C. 

38°  C. 
38-5°  C. 


II.  Effect  of  Respiration  on  the  Blood.— To  understand 
these  changes  in  the  air  we  must  refer  to  the  changes  in  the 
gases  of  the  blood  in  passing  through  the  lungs  (p.  199). 
Analyses  show  that  the  blood  going  to  the  lungs  is  poorer  in 
oxygen  and  richer  in  carbon  dioxide  than  the  blood  coming 


300 


HUMAN   PHYSIOLOGY 


FIG.  140.— Shows  the  Difference 
in  the  Gases  of  Arterial  and 
Venous  Blood. 


from  the  lungs  (Fig.  137).  Oxygen  is  taken  by  the  blood 
from  the  air,  carbon  dioxide  is  given  by  the  blood  to  the  air. 
How  is  this  effected  ?  The  extensive  capillary  network  in 
the  walls  of  the  air  vesicles,  if  spread  out  in  a  continuous 

sheet,  would  present  a  surface  of 
about  75  square  metres.  Between 
the  blood  in  the  capillaries  and 
the  air  in  the  air  vesicles  are  two 
layers  of  living  cells — 

1st.  The  endothelium  lining  the 
capillaries. 

2nd.  The  flattened  cells  lining 
the  air  vesicles.  Through  these  cells  the  interchange  of 
gases  must  take  place. 

This  interchange  might  take  place  in  two  different  ways — 
1st.  By  simple  mechanical  diffusion. 
2nd.  By  some  special  action  of  the  cells. 
If  the  process  follows  strictly  the  laws  of  simple  diffusion, 
it  is  then  unnecessary  to  invoke  the  activity  of  the  cells  as 
playing  a  part.     But  if  the  gaseous  interchange  does  not 
strictly  follow  these  laws,  we  must  conclude  that  the  cells 
do  play  a  part. 

Whether  a  gas  is  simply  dissolved  or  whether  it  be  held 
in  loose  chemical  combination,  the  amount  held  will  depend 
upon  the  temperature  of  the  fluid  and  upon  the  pressure  of 
the  gas  over  the  fluid.  If  the  temperature  is  raised  the 
fluid  will  hold  less  of  the  gas  in  solution,  and  any  chemical 
combination  will  tend  to  split,  as  is  seen  when  carbonate  of 
lime  is  heated  and  the  carbon  dioxide  is  driven  off. 

If  the  pressure  of  any  gas  over  a  fluid  be  decreased  the  gas 
will  tend  to  come  off  from  the  fluid,  if  it  is  increased  it  will 
be  taken  up  by  the  fluid.  This  is  well  seen  in  the  case  of 
soda  water.  But  the  same  law  applies  to  such  chemical 
compounds  as  carbonate  of  lime.  If  it  is  heated  in  ordinary 
air — i.e.  under  a  low  pressure  of  carbon  dioxide — the  gas  is 
given  off;  but  if  lime  is  in  an  atmosphere  of  C02  the  gas  does 
not  come  off,  but  is  taken  up  and  the  carbonate  is  formed. 

It  will  thus  be  seen  that  for  every  temperature  there  is  a 
certain  pressure  of  the  gas  at  which  the  solution  or  chemical 
combination  will  neither  give  off  nor  take  up  more  of  the  gas. 


THE  RESPIRATION  301 

This  may  be  determined  by  exposing  the  material  in  a  series 
of  chambers  to  air  containing  different  proportions  of  the  gas, 
and  ascertaining  by  analyses  of  the  air  whether  the  gas  has 
been  given  off  or  taken  up  or  has  remained  unaltered. 

We  know  that  the  pressure  of  gas  in  an  atmosphere 
depends  upon  the  proportion  present.  Suppose  an  atmos- 
phere contains  20  per  cent,  of  oxygen,  then  the  pressure,  or 
partial  pressure,  of  the  oxygen  is  got  by  multiplying  the 
percentage  amount  of  the  gas  by  760 — i.e.  a  whole  atmos- 
phere's pressure — and  dividing  by  100 — 

20x760 


100 


=  152  mm. 


By  ascertaining  at  what  partial  pressures  of  a  gas,  that  gas 
is  neither  given  off  nor  taken  up  by  a  fluid,  we  may  determine 
the  "  tension  "  of  the  gas  in  the  fluid  whether  it  be  in  solution 
or  in  loose  chemical  combination. 

The  presence  of  a  moist  membrane  between  the  fluid  and 
the  air  makes  no  difference  to  these  interchanges. 

These  facts  may  now  be  applied  to  the  question  as  to 
whether  the  gaseous  interchange  between  the  air  in  the 
air  vesicles  and  the  blood  is  due  simply  to  diffusion. 

The  questions  to  be  decided  are — 

1st.  What  is  the  partial  pressure  of  these  gases  in  the  blood 
going  to  and  coming  from  the  lungs  ? 

2nd.  What  is  their  partial  pressure  in  the  air  vesicles  ? 

Partial  pressure  of  Oxygen  and  Carbon  Dioxide  in  the 
Blood. — The  percentage  amount  of  these  gases  tells  us  nothing 
of  their  partial  pressure  since  they  are  in  chemical  combina- 
tion and  not  in  solution.  Experiments  made  upon  the 
subject  have  given  very  varying  results,  but  the  most  recent 
and  perfect  series  of  observations  go  to  show  that  the  partial 
pressure  of  oxygen  in  blood  coming  from  the  lungs — i.e.  in 
arterial  blood — is  about  100  mm.,  while  the  C02  pressure  in 
the  same  blood  varies  enormously,  but  on  an  average  is  about 
20  mm.  of  Hg. 

In  venous  blood  the  oxygen  pressure  must  be  much  lower — 
some  have  put  it  as  low  as  21  mm.  Hg ;  while  the  pressure 
of  C02  is  higher,  something  over  40  mm.  Hg. 

Partial  pressure  of  Gases  in  the  Air  Vesicle. — The  air  in 


302  HUMAN  PHYSIOLOGY 

the  alveoli  is  not  renewed  by  direct  ventilation  from  without, 
but  by  a  process  of  diffusion  (p.  287).  For  this  reason  the 
amount  of  oxygen  in  the  alveoli  must  be  much  smaller,  the 
amount  of  carbonic  acid  much  larger,  than  in  the  respired  air. 

By  catheterisation,  samples  of  air  have  been  withdrawn 
from  the  deeper  part  of  the  lungs  and  have  been  analysed. 

Such  analysis  tends  to  show  that  in  the  alveolar  air  there  is — 

Oxygen  about  10  per  cent,  at  a  partial  pressure  of  76 
mm.  Hg. 

Carbon  dioxide  about  4  per  cent,  at  a  partial  pressure  of 
about  30  mm.  Hg. 

The  difference  of  the  partial  pressure  of  these  gases  on  the 
two  sides  of  the  membrane — in  the  alveolar  air  and  in  the 
blood — may  be  represented  as  follows : — 


Oxygen, 
ir  Air,                  76 

/ 

Carbon  Dioxide. 
30 

* 

^ 

20? 
Venous 

100 

Arterial 

/ 
46 
Venous 

0—40 
Arterial 

Blood 

This  shows  that  when  the  blood  reaches  the  lungs  the 
distribution  of  pressure  in  the  gases  is  such  that,  by  the  laws 
of  diffusion,  oxygen  will  pass  from  the  alveolar  air  into  the 
blood  and  carbon  dioxide  from  blood  to  air ;  but,  before  the 
blood  has  left  the  lungs,  the  distribution  is  such  that  oxygen 
should,  by  the  laws  of  diffusion,  pass  from  blood  to  lungs 
and  carbon  dioxide  from  lungs  to  blood,  which  is  exactly  the 
reverse  of  what  occurs.  The  passage  of  oxygen  to  the  blood 
and  the  passage  of  carbon  dioxide  from  the  blood  is  much 
greater  than  could  be  accounted  for  by  diffusion. 

We  must,  therefore,  conclude  that  the  exchange  of  gases 
between  the  alveolar  air  and  the  blood  is  not  due  entirely 
to  diffusion,  but  is  in  part,  at  least,  brought  about  by  the 
activity  of  the  cells  lining  the  vessels  and  the  alveoli.  These 
conclusions  are  confirmed  by  the  observations  of  Haldane, 
that  when  air  containing  0*045  per  cent,  of  CO  is  shaken 
with  blood,  31  per  cent,  of  the  haemoglobin  is  combined  with 
that  gas,  while,  if  an  individual  breathes  such  air,  only  25  per 
cent,  of  the  haemoglobin  of  the  blood  is  combined.  In  some 


THE   RESPIRATION  303 

way  the  oxygen  of  the  air  is  passed  to  the  blood  more  rapidly 
than  the  CO. 

It  should,  however,  be  stated  that,  in  spite  of  these  figures, 
some  physiologists  maintain  that  simple  diffusion  will  explain 
these  interchanges,  1st,  because,  when  the  amount  of  oxygen 
in  the  atmosphere — i.e.  when  the  partial  pressure  of  O  falls 
below  a  certain  point,  the  gas  is  no  longer  taken  up  by  the 
blood,  and  2nd,  because,  when  the  amount  and  pressure  of 
C02  rises,  the  C02  is  not  given  off  from  the  blood. 

It  has  been  ascertained  by  experiment  that  the  pressure 
of  oxygen  in  the  air  may  fall  to  about  half  its  usual  pressure 
without  interfering  with  the  oxygenation  of  the  blood.  If  it 
falls  below  this  oxygen  is  not  taken  up.  Hence  it  is  possible 
for  men  to  live  at  high  altitudes  where  the  oxygen  tension 
is  much  reduced.  But  under  these  conditions  the  rate  of 
respirations  and  the  rate  of  blood  flow  through  the  lungs 
have  to  be  accelerated,  and  hence  hyperpnoea  is  apt  to  be 
induced,  especially  on  exertion. 

II.   INTERNAL  RESPIRATION. 

Having  seen  how  the  blood  gets  its  oxygen  and  gets  rid  of 
its  carbon  dioxide  in  the  lungs,  it  is  necessary  to  consider 
the  exchanges  of  these  gases  between  the  blood  and  the 
tissues. 

1st.  Passage  of  Oxygen  from  Blood  to  Tissues. 

In  studying  the  physiology  of  muscle,  which  may  be  taken 
as  a  type  of  all  the  active  tissues,  it  was  seen  that  oxygen 
is  constantly  being  built  up  into  the  muscle  molecule,  and 
that  the  living  tissues  have  such  an  affinity  for  oxygen  that 
they  can  split  it  off  from  such  pigments  as  methylene  blue. 
The  tension  of  oxygen  in  muscle  is  therefore  always  very 
low.  We  have  seen  that  the  tension  of  oxygen  in  arterial 
blood  is  nearly  100  mm.  Hg,  therefore,  when  the  blood  is 
exposed  to  a  low  tension  of  oxygen  in  the  tissues,  the  oxygen 
comes  off  from  the  blood  and  passes  into  the  tissues  by  the 
ordinary  laws  of  diffusion. 

But  it  must  be  remembered  that  this  takes  place  in  three 
stages. 

(1)  The  tissue  elements   are  always   taking   up  oxygen 


304  HUMAN  PHYSIOLOGY 

from  the  lymph,  because  of  the  very  low  pressure  of  oxygen 
in  the  protoplasm  and  because  the  protoplasm  has  an  affinity 
for  oxygen  as  for  other  nourishing  substances. 

(2)  As  a  result  of  this  the  oxygen  pressure  in  the  lymph 
falls  and  becomes  lower  than  the  oxygen  pressure  of  the 
blood  plasma,   and   thus   the  gas   passes   from    the    blood 
through   the  capillary  walls  to  the  lymph.     How  far  this 
is  simply  the  result  of  mechanical  diffusion,  and  how  far  it 
is  carried  on  by  the  vital  action  of  the  endothelium  of  the 
capillaries,  we  do  not  know. 

(3)  As  a  result  of  the  withdrawal  of  oxygen  from   the 
plasma,   the   partial    pressure    round    the    erythrocytes    is 
diminished,  and  the  blood  being  at  a  high  temperature,  a 
dissociation  of  oxyhsemoglobin  takes  place,  and  the  oxygen 
passes  out  into  the  plasma,  leaving  reduced  haemoglobin  in 
the  erythrocytes. 

2nd.  Passage  of  Carbon  Dioxide  from  Tissues  to  Blood. 

The  tissues  are  constantly  producing  carbon  dioxide.  In 
the  blood  the  carbon  dioxide  is  combined  with  the  soda  of 
the  plasma  and  is  thus  at  a  low  tension.  Hence  there  is  a 
constant  passage  of  carbonic  acid  from  the  tissues  to  the 
blood. 

III.  Extent  of  Respiratory  Interchange. 

The  extent  of  the  respiratory  interchange  in  the  lungs  is 
governed  by  the  extent  of  the  internal  respiratory  changes 
— i.e.  by  the  activity  of  the  tissues.  Every  factor  which 
increases  the  activity  of  the  metabolic  changes  in  the  tissues 
increases  the  intake  of  oxygen  and  output  of  carbon  dioxide 
by  the  lungs. 

Under  average  conditions  a  normal  man  of  65  kgs. 
excretes,  in  24  hours,  about  432  litres  (about  850  grms.)  of 
C02,  or  230  grms.  of  C,  and  takes  up  540  litres  of  0  (about 
770  grms.).  That  is,  in  one  hour,  on  an  average,  he  excretes 
20  litres  of  C02  and  absorbs  22  litres  of  0.  The  rate  of 
exchange  per  unit  of  weight  is  more  active  in  the  child  than 
in  the  adult. 

1.  Muscular  Work. — Since  muscle  is  the  most  abundant 
and  active  tissue  of  the  body,  muscular  work  more  than 
anything  else  increases  the  respiratory  changes  (see  p.  71). 


THE  RESPIRATION  305 

2.  Food. — The  taking  of  food  at  once  sets  up  active  changes 
in  the  digestive  apparatus.      The  muscular  mechanism  is 
set  in  action,  and  the  various  glands  secrete.     As  a  result 
of  these  processes  the  respiratory  interchange  is  at  once 
increased.      That   the   increase  is  dependent  upon   the  in- 
creased activity  is  shown  by  the  fact  that  it  is  produced 
by  the  taking  of  substances  which  cannot  be  absorbed  and 
used  in  the  metabolic  processes  of  the  body. 

But  while  the  immediate  increase  in  the  respiratory  inter- 
change following  the  taking  of  food  is  due  to  the  increased 
activity  of  the  digestive  structures,  there  is  also  an  increase 
due  to  the  utilisation  in  the  body  of  the  food  taken. 
Whether  proteids,  fats,  or  carbohydrates,  or  all  of  these  form 
the  diet,  more  oxygen  is  consumed  and  more  carbon  dioxide 
given  off  than  during  starvation.  The  proportion  between 
the  oxygen  taken  and  the  carbon  dioxide  excreted  is  not, 
however,  the  same  with  all  these  food-stuffs.  If  the  food 
is  rich  in  carbon  and  poor  in  oxygen,  a  greater  quantity  of 
oxygen  must  be  taken  to  oxidise  it  than  if  it  is  rich  in 
oxygen.  Thus  while  the  carbohydrates  contain  about  40  per 
cent,  of  C  and  53  per  cent,  of  O,  the  fats  contain  about  76 
per  cent,  of  C  and  only  12  per  cent,  of  0,  and  the  proteids 
contain  52  per  cent,  of  C  and  22  per  cent,  of  0. 

Hence  on  a  carbohydrate  diet   the  respiratory  quotient 

(p.   299)  £§*  is  high>  about  °'9  to   1*  wnile  on   a  fatty  or 
proteid  diet  it  is  low,  0-7  to  0-8. 

3.  Temperature. — If  the  temperature  of  the  body  is  in- 
creased, the  metabolic  processes  become  more  active   and 
the  respiratory  interchanges  are  increased.     But  if  the  tem- 
perature of  the  air  round  the  body  is  elevated,  the  metabolic 
processes  may  be  diminished  in  activity,  and  the  respiratory 
exchange  decreased. 

4.  Light. — It   has   been   shown   that   light   increases   the 
metabolic  changes  and  therefore  the  respiratory  activity. 

5.  Sleep. — Since  in  sleep  the  animal  is  in  a  condition  of 
muscular  rest,  since  light  is  excluded  from  the  eyes,  and 
since  food  is  not  taken,  the  respiratory  exchanges  are  less 
active  during  sleep  than  during  the  waking  hours.     Simi- 
larly, in  the  long  winter  sleep  of  certain  animals  (hiber- 
nation), these  factors  as  well  as  the  diminished  temperature 

20 


3o6  HUMAN  PHYSIOLOGY 

of  the  body  cause  a  great  reduction  in  the  intake  of  oxygen 
and  output  of  carbon  dioxide. 

It  is  thus  the  internal  which  governs  the  external  respira- 
tion. Merely  increasing  the  number  or  depth  of  the 
respirations  has  no  influence  on  the  amount  of  the 
respiratory  interchanges. 

Ventilation. 

The  rate  of  gaseous  exchange  governs  the  necessary 
supply  of  fresh  air.  It  has  been  found  that  if  the  supply 
is  insufficient,  headache  and  sleepiness  are  apt  to  supervene, 
and  experience  has  taught  that  each  adult  should  have 
2000  cubic  feet  of  fresh  air  per  hour.  If  1000  cubic  feet 
of  space  is  allowed  to  each  individual,  the  ordinary  methods 
of  exchange  of  air — through  the  chimneys  and  through 
chinks  in  windows  and  doors — should  supply  this  quantity 
of  air. 

Asphyxia. 

This  is  the  condition  caused  by  any  interference  with  the 
supply  of  oxygen  to  the  blood  and  tissues.  It  may  be 
induced  rapidly  and  in  an  acute  form  by  preventing  the 
entrance  of  air  to  the  lungs,  as  in  drowning  or  suffocation,  or 
by  causing  the  animal  to  breathe  air  deprived  of  oxygen,  or 
by  interfering  with  the  flow  of  blood  through  the  lungs,  or 
with  the  oxygen-carrying  capacity  of  the  blood.  It  is  slowly 
induced  in  a  less  acute  form  as  the  muscles  of  respiration 
fail  as  death  from  almost  any  disease  approaches. 

In  acute  asphyxia  there  is  an  initial  stage  of  increased 
respiratory  effort,  the  breathing  becoming  panting,  and  the 
expirations  more  and  more  forced.  The  pupils  are  small, 
and  the  heart  beats  more  slowly  and  more  forcibly,  while 
the  arterioles  are  strongly  contracted,  and  a  marked  rise 
in  the  arterial  pressure  is  produced.  Within  a  couple  of 
minutes  a  general  convulsion,  involving  chiefly  the  muscles 
of  expiration,  occurs.  The  intestinal  muscles  and  the 
muscles  of  the  bladder  may  be  stimulated,  and  the  faeces  and 
urine  may  be  passed  involuntarily.  Then  the  respirations 
stop,  deep  gasping  inspirations  occurring  at  longer  and  longer 
intervals.  The  pupils  are  dilated,  and  consciousness  is 


THE  RESPIRATION 


307 


abolished.  The  heart  fails,  and  thus,  although  the  arterioles 
are  still  contracted,  the  pressure  in  the  arteries  falls.  Finally 
the  movements  of  the  heart  cease  and  death  supervenes. 


VOICE. 

In  connection  with  the  respiratory  mechanism  of  many 
animals,  an  arrangement  for  the  production  of  sound  or  voice 
is  developed.  This  is  constructed  on  the  principle  of  a 
wind  instrument,  and  it  consists  of  a  bellows,  a  windpipe,  a 
vibrating  reed,  and  resonating 
chambers.  In  man  and  other 
mammals  the  bellows  are  formed 
by  the  lungs  and  the  thorax. 
The  trachea  is  the  windpipe. 
The  vocal  cords  in  the  larynx 
are  the  vibrating  reeds,  and  the 
resonating  chambers  are  the 
pharynx,  nose,  and  mouth. 

A.  Structure. — The  points  of 
physiological  importance  about 
the  structure  of  the  larynx  are 
the  following : — 

1.  Cartilages   (FigS.    141,   142).      FIG.  141.— Side  View  of  the  Carti- 

— The  ring-like  cricoid  (Cr.)  at 
the  top  of  the  trachea  is  thickened 
from  below  upwards  at  its  pos- 
terior part  and  carries  on  its 
upper  border  two  pyramidal  car- 
tilages triangular  in  section — the  arytenoids  (Ar.).  These 
articulate  with  the  cricoid  by  their  inner  angle.  At  the 
outer  angle  the  posterior  and  lateral  crico-arytenoid 
muscles  are  attached.  From  their  anterior  angles  the 
vocal  cords  arise  and  run  forward  to  the  thyroid.  The 
thyroid  cartilage  (Th.)  forms  a  large  shield  which  articu- 
lates by  its  posterior  and  inferior  process  with  the  sides  of 
the  cricoid  so  that  it  moves  round  a  horizontal  axis.  To 
the  upper  and  anterior  part,  the  epiglottis  or  cartilaginous 
lid  of  the  larynx  is  fixed. 

2.  Ligaments. — The  articular  ligaments  require  no  special 


Cr. 


lages  of  the  Larynx.  Cr.,  Cri- 
coid Cartilage;  Ar.,  Right 
Arytenoid  Cartilage ;  Th. ,  Thy- 
roid Cartilage.  The  dotted  line 
shows  the  change  in  the  position 
of  the  Thyroid  by  the  action  of 
the  Crico-thyroid  Muscle. 


308  HUMAN  PHYSIOLOGY 

attention.  The  true  vocal  cords  are  fibrous  ligamentous 
structures  which  run  from  the  arytenoids  forward  to  the 
posterior  aspect  of  the  middle  of  the  thyroid.  They  contain 
many  elastic  fibres  and  are  covered  by  a  stratified  squamous 
epithelium  and  appear  white  and  shining. 

The  vocal  cords  increase  in  length  as  the  larynx  grows, 
and  in  adult  life  they  are  generally  longer  in  the  male  than 
in  the  female,  and  the  whole  larynx  is  larger. 

3.  Muscles. — The  crico-thyroids  take  origin  from  the 
antero-lateral  aspects  of  the  cricoid,  and  are  inserted  into 
the  inferior  part  of  the  lateral  aspect  of  the  thyroid.  In 
contracting  they  approximate  the  two  cartilages  anteriorly, 
and  render  tense  the  vocal  cords  (Fig.  141). 

The  crico-arytenoidei  postici  arise  from  the  back  of  the 
cricoid  and  pass  outwards  to  be  inserted  into  the  external 
or  muscular  process  of  the  arytenoids.  In  contracting 
they  pull  these  processes  inwards,  and  thus  diverge  the 
anterior  processes  and  open  the  glottis  (Fig.  142). 

The  crico-arytenoidei  laterales  take  origin  from  the  lateral 
aspects  of  the  cricoid,  and  pass  backwards  to  be  inserted  into 
the  muscular  processes  of  the  arytenoids.  They  pull  these 
forward  and  so  swing  inwards  their  anterior  processes,  and 
approximate  the  vocal  cords  (Fig.  142). 

A  set  of  muscular  fibres  run  between  the  arytenoids — the 
arytenoidei — while  other  fibres  run  from  the  arytenoids  up 
to  the  side  of  the  epiglottis.  These  help  to  close  the  upper 
orifice  of  the  larynx. 

The  thyro-arytenoid  is  a  band  of  muscular  fibres  lying 
in  the  vocal  cords  and  running  from  the  thyroid  to  the 
arytenoids.  Its  mode  of  action  is  not  fully  understood. 

5.  Mucous  Membrane. — The  mucous  membrane  of  the 
larynx  is  raised  on  each  side  into  a  well-marked  fold 
above  each  true  vocal  cord — the  false  vocal  cord.  Be- 
tween this  and  the  true  cord  is  a  cavity — the  ventricle 
of  the  larynx.  The  other  folds  of  mucous  membrane, 
although  of  importance  in  medicine,  have  no  special  physio- 
logical significance. 

The  interior  of  the  larynx  may  be  examined  during  life  by 
the  laryngoscope.  (Practical  Exercise.) 

5.  Nerves. — The  muscles  of  the  larynx  are  supplied  chiefly 


THE  RESPIRATION 


309 


by  the  recurrent  laryngeal  branch  of  the  vagus  which  comes 
off  in  the  thorax  and  arches  upwards  to  the  larynx.  On  the 
left  side,  where  it  curves  round  the  aorta,  it  is  apt  to  be 
pressed  upon  in  aneurismal  swellings.  Paralysis  of  this 
nerve  causes  the  vocal  cord  on  that  side  to  assume  the 
cadaveric  position,  midway  between  adduction  and  abduc- 
tion, and  makes  the  voice  hoarse  or  abolishes  it  altogether. 
The  superior  laryngeal  is  the  great  ingoing  nerve,  but  it 
also  supplies  motor  fibres 
to  the  crico-thyroid  muscle. 
Paralysis  prevents  the 
stretching  of  the  vocal  cords, 
makes  the  voice  hoarse  and 
renders  it  impossible  to 
produce  a  high  note. 

Centre. — These  nerves  are 
presided  over  by  (a)  a  centre 
in  the  medulla.  When  this 
is  stimulated  abduction  of  the 
vocal  cords  is  brought  about. 
(6)  This  centre  is  controlled 
by  a  cortical  centre  situated 
in  the  inferior  frontal  con- 
volution. Stimulation  of 
this  causes  adduction  of  the 
cords  as  in  phonation,  while 
destruction  leads  to  no 


Cr. 


Ar. 


marked  change. 


FIG.  142. — Cross  Section  of  the  Larynx, 
to  show  the  Cricoid,  Cr.  ;  Thyroid, 
Th. ;  Arytenoid  Cartilages,  Ar.  The 
continuous  line  shows  the  parts  at 
rest.  The  dotted  line  under  the  action 
of  the  Lateral  Crico-arytenoid  Muscle, 
and  the  dot-dash  line  under  the  action 
of  the  Posterior  Crico-arytenoid. 

B.  Physiology.  —  When  a 

blast  of  air  is  forced  between  the  vocal  cords  they  are  set 
in  vibration  both  wholly  and  in  segments  like  other 
vibrating  reeds,  and  sounds  are  thus  produced.  These 
sounds  may  be  varied  in  loudness,  pitch,  and  quality. 

The  loudness,  or  amplitude  of  vibration,  depends  upon  the 
size  of  the  larynx  and  upon  the  force  of  the  blast  of  air 
acting  on  the  cords. 

The  pitch  or  number  of  vibrations  per  second  depends 
upon  the  length  and  tension  of  the  vocal  cords.  The  greater 
length  of  the  vocal  cords  in  the  male,  as  compared  with  the 


3io 


HUMAN  PHYSIOLOGY 


female,  makes  the  voice  deeper.  The  tension  of  the  cords 
may  be  varied  by  the  action  of  the  crico- thyroid  muscle. 

The  power  of  varying  the  pitch  of  the  voice  differs  greatly 
in  different  people.  The  average  difference  between  the 
lowest  and  the  highest  note  which  the  ordinary  individual 
can  produce  is  about  two  octaves. 

The  quality  of  the  sound  depends  upon  the  overtones 
which  are  made  prominent  by  resonance  in  the  pharynx, 
nose,  and  mouth.  By  varying  the  shape  and  size  of  these 
cavities  and  more  especially  of  the  mouth,  the  quality  of 
sound  may  be  considerably  varied. 

Singing  Voice. — This  is  often  classified  by  its  average  pitch 


A  i  u 

FIG.  143.— Changes  in  the  Shape  of  the  Mouth  in  Sounding  the  Vowels  A,  7,  U. 


and  by  its  quality  into  bass  and  tenor  in  males,  and  contralto 
and  soprano  in  females. 

The  true  or  chest  voice  is  produced  by  a  blast  of  air  from 
the  chest  setting  the  cords  into  vibration,  but  the  falsetto 
voice,  which  is  generally  higher  in  pitch  than  the  true 
voice,  is  probably  produced  by  vibrations  of  the  edges  of 
the  cords. 

Articulate  Speech. — Spoken  language  is  produced  by 
varying  the  resonating  chambers  of  the  pharynx,  nose,  and 
mouth,  and  thus  modifying  the  sounds  produced  in  the 
larynx.  Whispered  speech  is  produced  by  setting  in  vibra- 
tion in  the  resonating  chambers  a  stream  of  air  which  has 
not  been  allowed  to  act  upon  the  vocal  cords. 

In  articulate  speech  two  classes  of  sounds  may  be  dis- 


THE  RESPIRATION 


tinguished — (1)  Yowel  Sounds.  These  are  musical  sounds 
produced  by  a  blast  of  air  which  is  so  modified  by  changing 
the  shape  of  the  mouth  as  to  produce  the  well-known  a,  e,  i, 
o,  u  (Fig.  143). 

In  sounding  a,  the  mouth  resembles  a  funnel  with  the 
wide  part  forward. 

In  i,  it  may  be  compared  to  a  flask  with  the  belly  behind 
and  the  neck  forward. 

In  u,  the  flask  is  reversed,  the  belly  being  forward. 

In  o,  the  mouth  is  intermediate  between  its  position  in 
u  and  a,  and  in  e  between  a  and  i. 

(2)  Consonant  Sounds. — These  are  more  of  the  nature  of 
noises — irregular  vibration.  They  are  produced  either — (1) 
at  the  lips,  (2)  between  the  teeth  and  hard  palate  and 
the  tongue,  or  (3)  at  the  soft  palate  and  back  of  the  tongue. 
At  these  situations  sounds  may  be  produced — (1)  by  closing 
the  orifice  and  then  suddenly  forcing  it  open,  (2)  by  sending 
a  current  of  air  over  a  narrowing  produced  at  one  of  these 
places,  (3)  by  setting  the  edges  of  the  narrowing  in  vibration. 
When  the  mouth  is  closed  at  one  of  these  situations  and  air 
is  forced  through  the  nose  the  nasal  consonant  sounds  result. 

Thus,  according  to  their  mode  of  production,  the  consonant 
sounds  may  be  classified  into  explosives,  aspirates,  vibratories, 
or  nasals. 


Labials. 

Dentals. 

Gutturals. 

Explosives  . 
Aspirates    . 
Vibratory   . 
Nasals 

P.  B. 

F.  V. 

M. 

T.  D. 
S.  L.  Th. 
R.  (English). 

•j/-      pi 

Ch.  (German). 
R.  (French). 

Ng. 

SECTION    IX 
THE  FOOD  AND  DIGESTION 

I.  FOOD 

The  great  use  of  food  is  to  supply  energy  to  the  body.  The 
muscles  of  the  body  are  constantly  active;  they  are  con- 
stantly liberating  energy  by  breaking  down  the  complex 
molecules  of  proteids,  fats,  and  carbohydrates,  and  hence  a 
constant  fresh  supply  of  such  material  is  necessary  to  pre- 
vent the  body  living  on  its  own  material  and  wasting  away 
(see  p.  73).  During  the  growth  of  the  body,  too,  the  material 
from  which  it  is  to  be  built  up,  and  the  energy  used  in  its 
construction,  must  be  supplied  by  the  food. 

Hence  a  suitable  food  is  one  which  will  yield  the  necessary 
amount  of  energy,  and  will  supply  the  materials  necessary 
for  repair  and  for  growth. 

But  food  must  also  supply  the  water  and  salts  required  to 
keep  the  various  constituents  of  the  body  in  solution,  so  that 
the  essential  chemical  changes  may  go  on. 

Nature  of  Food. 

Food  may  be  divided  into  foods  not  yielding  energy  and 
foods  yielding  energy — the  former  acting  chiefly  as  solvents. 

A.  Food-stuffs  not  yielding  Energy. — 1.  Water  is  the 
chief  constituent.  Since  it  is  daily  given  off  in  large 
quantities  by  the  kidneys,  lungs,  skin,  and  bowels,  it  must 
daily  be  supplied  in  sufficient  amounts  or  the  chemical 
changes  cannot  go  on  and  death  supervenes. 

2.  Inorganic  salts. — The  most  important  of  these  is 
chloride  of  sodium,  which  is  essential  for  the  maintenance  of 
the  chemical  changes  in  the  body.  When  it  is  not  freely 


THE   FOOD   AND  DIGESTION  313 

supplied  in  the  food,  it  is  retained  in  the  tissues,  and  hence 
animals  can,  when  necessary,  live  with  a  comparatively  small 
supply. 

3.  Salts  of  organic  acids. — The  sodium  and  potassium 
salts  of  citric,  malic,  and  tartaric  acid,  which  are  found 
abundantly  in  various  vegetables,  are,  when  taken  into  the 
tissues,  oxidised  into  carbonates  which  are  strongly  alkaline 
salts.  The  proteids  which  are  decomposed  in  the  body 
contain  sulphur  and  phosphorus,  and  these  are  oxidised  into 
sulphuric  and  phosphoric  acids.  In  herbivorous  animals 
the  prejudicial  effect  of  such  acids  is  counteracted  by  the  for- 
mation of  these  alkaline  carbonates,  which  neutralise  the  acids. 
In  carnivorous  animals  these  salts  are  not  so  necessary,  since 
ammonium  is  formed  from  the  nitrogen  of  the  proteids  in 
sufficient  quantity  to  neutralise  the  acids  produced.  Man 
occupies  a  position  midway  between  the  herbivora  and  the 
carnivora.  The  amount  of  energy  yielded  by  the  breaking 
down  of  these  salts  into  carbonates  is  so  small  that  it  is  of  no 
importance. 

B.  Food-stuffs  yielding  Energy.  —  These  are  complex 
combinations  of  carbon,  hydrogen,  and  oxygen,  with  or 
without  nitrogen,  sulphur,  phosphorus,  and  iron.  They  are 
of  the  same  nature  as  the  materials  which  are  found  on 
analysis  of  dead  protoplasm.  They  are  commonly  spoken  of 
as  the  Proximate  Principles  of  the  food,  and  they  may  be 
classified  as  follows : — 

1.  Nitrogen-containing  —  Proteids    and    Albuminoids    or 
Modified  Proteids. 

2.  Non-nitrogenous — Fats  and  Carbohydrates. 

In  studying  the  value  of  these  food-stuffs  it  is  necessary  to 
consider  their  Energy  Yalue — that  is,  the  amount  of  energy 
which  can  be  yielded  by  the  decomposition  of  a  definite 
quantity  of  each  in  the  body. 

The  fats  and  carbohydrates  leave  the  body  as  carbon 
dioxide  and  water,  the  proteids  leave  it  partly  as  carbon 
dioxide  and  water,  partly  as  urea. 

Such  a  body  as  glucose,  C6H1206,  by  being  oxidised  to  C02 
and  H20,  gives  off'  a  certain  amount  of  kinetic  energy,  and 
the  amount  of  energy  liberated  is  the  same  whether  the 


3i4  HUMAN  PHYSIOLOGY 

oxidation  is  direct  or  takes  place  through  any  of  many 
possible  lines  of  chemical  change. 

In  whatever  ways  a  chemical  substance  breaks  down  into 
certain  final  products  the  energy  set  free  is  always  the  same, 
and  this  principle  is  taken  advantage  of  in  determining  the 
energy  value  of  the  food-stuffs.  If  fats  and  carbohydrates 
are  changed  to  carbon  dioxide  and  water  in  the  body,  and  if 
the  energy  given  off  as  they  undergo  this  change  can  be 
measured  outside  the  body,  their  energy  value  as  foods  can 
be  ascertained. 

Determination  of  Energy  Value. — This  is  done  by  burning 
a  definite  quantity  of  the  material  and  finding  how  many 
degrees  a  definite  quantity  of  water  is  heated.  This  gives 
the  energy  in  heat  units,  and,  by  Joule's  law,  it  can  be  con- 
verted into  the  equivalent  for  any  other  kind  of  energy,  such 
as  mechanical  work. 

It  is  known  that  the  energy  required  to  heat  one  kilo- 
gramme of  water  through  one  degree  Centigrade  is  sufficient 
to  raise  423  kilogrammes  of  matter  to  the  height  of  one 
metre  against  the  force  of  gravity,  and  thus  if  the  energy 
value  of  any  material  as  a  producer  of  heat  is  known  in 
heat  units — kilogramme  degrees  or  Calories — by  multiplying 
by  423  we  get  the  value  in  work  units — kilogramme  metres 
(Kgms.). 

The  apparatus  used  for  ascertaining  the  heat  produced  by 
the  combustion  of  material  is  called  a  calorimeter.  Many 
different  forms  are  in  use,  but  the  object  in  the  water  calori- 
meter is  to  secure  that  all  the  heat  is  used  in  raising  the 
temperature  of  a  known  volume  of  water. 

The  value  of  the  three  great  proximate  principles  of  the 
food  must  be  considered  in  detail. 

1.  Proteids. — The  chemistry  of  these  bodies  has  been 
already  considered  (pp.  9  and  10).  They  are  the  "  chief  sub- 
stances" of  living  matter,  forming  about  80  or  90  per  cent, 
of  its  dry  residue.  The  molecule  is  one  of  great  complexity, 
and  contains  C.H.O.N.S.,  and  sometimes  P.  and  Fe. 

It  is  from  the  proteids  of  the  food  alone  that  the  nitrogen 
and  sulphur  required  in  the  construction  and  repair  of  the 
living  tissues  are  obtained.  The  carbon  and  hydrogen 


THE   FOOD   AND  DIGESTION  315 

required  are  also  contained  in  these  substances;  and,  as 
will  be  presently  shown,  they  have  a  considerable  energy 
value.  Hence  Proteids  form  THE  essential  organic  con- 
stituent of  the  food.  Theoretically  it  should  be  perfectly 
possible  for  an  animal  to  live  on  proteids  alone,  with  a 
suitable  addition  of  water  and  salts. 

In  estimating  the  actual  energy  value  of  proteids  in  the 
body  a  difficulty  arises  in  the  fact  that,  instead  of  being 
decomposed  to  C02,  H90,  NH3,  S03,  as  they  are  during  com- 
bustion, in  the  body  the  nitrogenous  part  is  not  broken  down 
further  than  urea — CON2H4,  3  grms.  of  proteid  yielding  1 
grm.  of  urea.  If  the  energy  value  of  the  complete  combus- 
tion of  a  definite  amount  of  proteids  is  first  ascertained,  and 
then  the  energy  value  of  the  amount  of  urea  derived  from 
the  same  amount  of  proteid  is  determined,  by  subtract- 
ing the  latter  from  the  former,  the  energy  value  of  proteids 
in  their  decomposition  in  the  body  is  found  (see  p.  318). 
The  combustion  of  1  grm.  of  proteid  to  urea  yields  4*1 
Calories  of  Energy. 

2.  Modified    Proteids    (Albuminoids). — In     studying    the 
chemistry  of  the  formed  material  of  the  various  protecting, 
supporting,  and  connecting  tissues,  these   substances  have 
been  considered  (p.  29). 

Keratin,  elastin,  and  mucin  seem  to  be  of  no  importance 
as  articles  of  food.  If  taken  in  the  food  they  pass  through 
the  alimentary  canal  practically  unchanged. 

While  raw  collagen  seems  also  to  be  of  little  use,  gelatin, 
formed  by  boiling  collagen,  has  a  certain  value.  Although 
it  cannot  take  the  place  of  proteids,  because  it  cannot  be 
used  for  building  up  the  living  tissues  of  the  body,  it  is 
nevertheless  decomposed  into  urea,  and  in  decomposing  it 
yields  the  same  amount  of  energy  as  the  proteids.  It  has, 
therefore,  a  definite  though  restricted  value  as  a  food 
stuff. 

3.  Fats. — The  chemistry  of  the  fats  has  already  been  con- 
sidered (p.  31).     From  the  fact  that  they  contain  so  little 
oxygen  in  proportion  to  their  carbon  and  hydrogen,  a  large 
amount   of  energy  is  liberated   in    their    combustion,   and 
therefore  they  have  a  high  energy  value  as  food-stuffs. 

One  gramme  of  fat  yields  twice  as  much  energy  as  the 


3i6  HUMAN   PHYSIOLOGY 

same  amount  of  proteid  or  carbohydrate.  The  combustion 
of  1  gramme  of  fat  yields  9-3  Calories  of  Energy. 

4.  Carbohydrates  (for  tests  for  different  carbohydrates, 
see  "  Chemical  Physiology,"  p.  8,  et  seq.). — The  carbohydrates 
— starches  and  sugars — form  a  group  of  bodies  which  do  not 
occur  largely  in  animals,  but  are  abundant  constituents  of 
plants. 

They  contain  carbon,  hydrogen,  and  oxygen,  the  carbon 
atoms  of  the  molecule  usually  numbering  six  or  some  mul- 
tiple of  six,  and  the  hydrogen  and  oxygen  being  in  the  same 
proportions  in  which  they  occur  in  water.  They  are 
aldehydes  or  ketones,  and  derivatives  from  these,  of  the 
hexatomic  alcohol,  CCHUO6  (p.  422).  A  group  of  carbo- 
hydrates having  five  carbon  atoms,  and  hence  called  Pentoses, 
have  been  found  in  the  animal  body,  but  they  are  of  minor 
importance. 

The  simplest  carbohydrates  are  the  monosaccharids,  of 
which  dextrose,  the  aldehyde  of  mannite,  is  the  most  im- 
portant. Dextrose  is  the  sugar  of  the  animal  body.  It 
has  been  called  glucose,  grape  sugar,  and  blood  sugar. 

Closely  allied  to  dextrose  in  chemical  composition  is 
kevulose,  a  sugar  which,  instead  of  rotating  the  plane  of 
polarised  light  to  the  right,  rotates  it  to  the  left,  but  which 
in  other  respects  behaves  like  dextrose.  It  occurs  in  certain 
plants. 

The  other  monosaccharid  of  importance  is  galactose,  a 
sugar  produced  by  the  splitting  of  milk  sugar. 

These  monosaccharids,  when  boiled  with  a  solution  of 
cupric  acetate  in  acetic  acid  (Barfoed's  solution),  are  oxidised, 
taking  oxygen  from  the  cupric  salt  and  reducing  it  to  the 
cuprous  state.  When  boiled  with  caustic  potash,  they,  along 
with  certain  of  the  double  sugars,  are  oxidised,  and  if  a 
metallic  salt  be  present  which  can  readily  give  up  its  oxygen 
it  becomes  reduced,  the  sugar  appropriating  the  oxygen. 
On  this  depends  Fehling's  and  many  other  tests  for  glucose. 

Under  the  influence  of  yeast  they  split  into  ethyl  alcohol 
and  carbon  dioxide. 

They  also  form  crystalline  compounds,  osazones,  with 
phenylhydrazin.  These  have  proved  most  useful  in  dis- 
tinguishing different  sugars. 


THE   FOOD  AND   DIGESTION 


By  the  polymerisation  of  two  monosaccharid  molecules 
with  the  loss  of  water,  disaccharids,  or  double  sugars,  are 
formed.  Thus,  two  glucose  molecules  polymerise  to  form 
one  maltose  molecule. 

1206  =  2  (c6H1206)-H20 


Maltose  is  the  sugar  formed  by  the  action  of  malt  and  other 
vegetable  and  animal  zymins  upon  starch.  By  the  action  of 
dilute  acids  and  other  agents  it  can  be  split  into  two  dextrose 
molecules.  Like  the  monosaccharids,  it  ferments  with  yeast. 

Lactose,  the  sugar  of  milk,  is  a  disaccharid  composed  of 
a  molecule  of  dextrose  united  to  a  molecule  of  galactose 
with  dehydration.  It  readily  splits  into  these  two  mono- 
saccharids, but  does  not  ferment  with  yeast. 

Dextrose,  polymerising  with  laevulose,  yields  cane  sugar, 
and  this  sugar,  so  largely  used  as  an  article  of  food,  can  be 
split  into  dextrose  and  lasvulose.  It  does  not  reduce  Fehling's 
solution,  and  does  not  ferment  with  yeast. 

By  further  polymerisation  of  monosaccharids  with  the  loss 
of  water,  molecules  of  greater  size  are  produced  and  form  the 
set  of  substances  known  as  the  polysaccharids.  Among  the 
simplest  of  these  is  dextrin,  produced  by  the  polymerisation 
of  twelve  molecules  of  glucose  with  the  loss  of  twelve 
molecules  of  water. 

12(C6H1206-H20) 
=  12(C6H1005) 

Closely  allied  to  dextrin  is  inulin.  But  while  dextrin  is 
formed  of  dextrose  molecules,  inulin  contains  Isevulose  mole- 
cules. Both  are  formed  from  the  splitting  of  the  more 
complex  starches.  The  molecule  of  soluble  starch  is  built  up 
of  no  less  than  thirty  dehydrated  monosaccharid  molecules, 
and  has  a  molecular  weight  of  9000.  Ordinary  starch  seems 
to  have  a  molecular  weight  of  20,000  or  30,000,  .and  hence 
must  be  of  still  greater  complexity. 

These  polysaccharids  are  distinguished  from  the  sugars  by 
being  precipitated  from  their  solutions  by  the  addition  of 
alcohol.  They  are  not  oxidised  when  boiled  with  caustic 
potash,  nor  do  they  change  to  alcohol  and  carbon  dioxide 
under  the  influence  of  yeast.  In  cold  neutral  or  acid  solu- 


3i8  HUMAN  PHYSIOLOGY 

tions  most  of  them  strike  a  blue  or  brown  colour  with  iodine. 
By  boiling  with  a  mineral  acid  and  by  the  influence  of  various 
ferments  they  break  down,  take  up  water,  and  become  mono- 
saccharids — the  starches  yielding  the  dextroses,  and  inulins 
yielding  Isevulose. 

Glycogen  is  animal  starch.  It  gives  an  opalescent  solution 
and  strikes  a  brown  with  iodine. 

The  energy  value  of  the  carbohydrates  is  about  the  same 
as  that  of  the  proteids. 

Energy  Yalue  of  the  Proximate  Principles  of  the  Food.— 
1  gramme  of — 

Proteid  yields     .        .        .        .4-1  Calories. 
Carbohydrate  yields  .         .        .4-1 
Fat  yields  ....       9-3        „ 

The  Sources  of  the  Various  Proximate  Principles  of 
the  Food. 

The  proximate  principles  are  in  part  derived  from  the 
animal,  in  part  from  the  vegetable  kingdoms.  While  some 
races  procure  their  food  entirely,  or  almost  entirely,  from  the 
former,  others  depend  almost  entirely  on  the  latter.  The 
vast  majority  of  mankind,  however,  use  a  mixture  of  animal 
and  vegetable  foods. 

Animal  foods  may  be  classified  as— 

1.  Milk  and  its  Products,  Cream,  Butter,  and  Cheese. 

2.  Flesh. 

3.  Eggs. 

1.  Milk. — The  characters  of  human  milk  are  considered  at 
p.  411.  Cow's  milk  is  an  important  constituent  of  the  diet. 
Its  average  composition  compared  with  human  milk  is  as 
follows : — 


Cow's. 

Human. 

Water         .... 

88-3 

88-8 

Proteids      ... 

3-0 

1-0 

Fats    

3'5 

3'5 

Carbohydrates    . 

4-5                   6'5 

Salts  

0-7 

0-2 

THE   FOOD  AND   DIGESTION  319 

The  chief  proteid  of  milk  is  caseinogen,  a  nucleo-proteid 
with  a  very  small  amount  of  phosphorus,  which  exists  as  a 
soluble  calcic  compound.  It  is  held  in  solution  in  milk,  but 
under  the  influence  of  various  agents  it  clots  or  curds.  It  is 
split  by  the  action  of  acids,  and  the  casein  is  precipitated. 
Under  the  influence  of  rennet  it  is  also  split  into  whey 
albumin  which  remains  in  solution,  and  calcic  paracasein 
which  is  insoluble.  In  cow's  milk  a  small  amount  of  an 
albumin  is  also  present. 

The  fats  of  milk  occur  as  small  globules  of  varying  size 
floating  in  the  fluid,  each  surrounded  by  a  proteid  envelope 
which  must  be  removed  by  means  of  an  alkali  or  an  acid 
before  the  fat  can  be  extracted  with  ether.  The  fats  are 
chiefly  olein  with  smaller  quantities  of  palmatin  and  stearin, 
and  still  smaller  amounts  of  such  lower  fats  as  butyrin, 
capronin,  and  caprylin. 

The  carbohydrate  of  milk  is  lactose,  a  disaccharid,  which 
splits  into  dextrose  and  galactose. 

The  ash  of  milk  is  rich  in  phosphoric  acid,  calcium,  and 
potassium — poor  in  sodium  and  iron. 

Butter  and  Cream  are  simply  the  fats  of  the  milk  more  or 
less  completely  separated  from  the  other  constituents. 

Cheese  is  produced  by  causing  the  coagulation  of  the 
casein,  which  carries  with  it  a  large  amount  of  the  fats.  If 
cheese  is  made  before  the  removal  of  the  cream  it  is  rich  in 
fats,  if  after  the  removal  of  the  cream  it  is  poor  in  fats. 
Cheese  contains  between  25  and  30  per  cent,  of  proteid,  and 
between  10  and  30  per  cent,  of  fat.  It  is  as  a  source  of 
proteid  that  it  is  of  chief  value. 

Cheese,  when  allowed  to  stand,  affords  a  suitable  nidus 
for  the  growth  of  micro-organisms  by  the  action  of  which 
the  proteids  are  digested  into  peptones  and  simpler  bodies, 
and  the  fats  split  up  into  glycerine  and  the  lower  fatty 
acids.  These  free  fatty  acids  give  the  peculiar  flavour  to 
ripe  cheese.  The  lactose  is  in  part  converted  into  lactic 
acid. 

2.  Flesh. — Under  this  head  may  be  included  not  only  the 
muscles  of  various  animals,  but  also  such  cellular  organs  as 
the  liver  and  kidneys. 

When  free  of  fat  they  contain  about  20  per  cent,  of  pro- 


320  HUMAN  PHYSIOLOGY 

teids.  The  amount  of  fat  may  vary  from  almost  nil  in  white 
fish  to  about  80  per  cent,  in  fat  bacon. 

Flesh  is  thus  a  source  of  proteids  and  albuminoids,  and  to 
a  smaller  extent  of  fats.  In  animals  specially  fed  the  amount 
of  fat  may  be  enormously  increased,  and  ordinary  butchers' 
meat  may  have  more  fat  than  proteid.  The  extractives 
include  such  bodies  as  creatin,  xanthin,  inosit,  &c.  (see  p.  43), 
which  give  the  peculiar  flavour  to  the  flesh  of  various  animals. 
Flesh  may  be  preserved  in  various  ways — e.g.  by  simply 
drying,  by  salting,  or  by  smoking.  The  result  of  each  of 
these  procedures  is  to  dimmish  the  amount  of  water,  and 
thus  to  increase  the  solids. 

3.  Eggs. — The  egg  of  the  domestic  fowl  need  alone  be 
considered.  The  composition  of  the  white  and  of  the  yolk 
naturally  differs  considerably.  The  white  of  egg  is  nothing 
more  than  a  solution  of  proteids. 

In  the  yolk  there  is  a  very  large  amount  of  lecithin  (p.  78) 
along  with  ordinary  fats,  and  a  large  amount  of  a  phospho- 
proteid,  and  the  great  value  of  eggs  is  thus  that  they  con- 
tain both  proteids,  ordinary  fats,  and  this  special  fat.  The 
whole  egg  contains  a  little  more  than  10  per  cent,  each 
of  proteids  and  of  fats. 

Speaking  generally,  we  may  say  that  the  animal  food- 
stuff's are  rich  in  proteids  and  fats,  but  are  poor  in  carbo- 
hydrates. 

Vegetable  Food-stuffs. — In  the  food  of  man  vegetables 
play  as  important  a  part  as  animal  products. 

The  peculiarity  of  special  importance  in  vegetables  is  the 
existence  of  a  capsule  to  the  cells,  composed  of  cellulose— a 
substance  allied  to  starch  in  its  composition,  but  which  is 
very  resistant  to  the  action  of  the  human  digestive  juices, 
and  thus  hinders  the  utilisation  of  the  cell  contents.  In 
order  that  these  may  be  digested  and  absorbed  from  the 
stomach  and  intestine,  this  capsule  must  be  broken  down 
either  by  some  preparatory  treatment,  or  by  the  teeth  in  the 
act  of  chewing.  Although  of  practically  no  value  as  a  food- 
stuff, it  acts  as  a  natural  purgative  by  stimulating  the  intes- 
tines, and  is  of  great  value  in  keeping  up  the  regular  action 
of  the  bowels. 


THE  FOOD  AND  DIGESTION  321 

I.  Cereals. — From  the  seeds  of  these,  meals  and  flours  are 
prepared. 

Oatmeal  contains  about  15  per  cent,  of  proteids,  about 
6  per  cent,  of  fats,  and  about  65  per  cent,  of  carbohydrates. 

Wheaten  flour  contains  about  10  per  cent,  of  proteid,  1  per 
cent,  of  fat,  and  75  per  cent,  of  carbohydrates. 

Ordinary  white  bread,  prepared  from  wheaten  flour,  con- 
tains only  about  7  per  cent,  of  proteids  and  55  per  cent,  of 
carbohydrates. 

The  proteids  of  these  cereals  are  mixtures  of  various 
albumins  and  globulins  which  do  not  differ  in  their  char- 
acters from  the  animal  proteids.  They  are  most  abundant 
in  the  outer  part  of  the  grain,  just  under  the  capsule. 

While  oatmeal  and  maize  contain  a  fair  proportion  of  fat, 
the  other  cereals  are  poor  in  this  constituent. 

The  chief  constituents  of  all  these  seeds  are  the  carbo- 
hydrates stored  as  starch. 

II.  Legumens. — The  seeds  of  the  leguminosse — peas,  beans, 
and  lentils — are  valuable  constituents  of  the  diet,  being  speci- 
ally rich  in  proteids.     In  the  dry  state  they  contain  some- 
thing over  20  per  cent,  of  proteid  and  about  50  per  cent,  of 
carbohydrates. 

The  proteids  are  a  mixture  of  albumin  and  globulin,  which 
have  been  classified  together  as  legumin. 

The  fats  are  small  in  amount,  and  the  carbohydrates, 
though  abundant,  are  less  so  than  in  the  cereals. 

III.  Bulbous  Plants. — The  underground  stems  of  certain 
plants  develop  tuberous  growths  in  which  material  for  the 
nourishment  of  the  plant  is  stored.     These  plants  belong  to 
different  natural  orders,  but  they  may  here  be  classified  to- 
gether.    The  most  commonly  employed  are  potatoes,  turnips, 
and    carrots.      The   amount  of  proteid  is  small,  something 
under  2  per  cent.,  while  the  carbohydrates  in  the  potato  reach 
20  per  cent.,  but  in  other  tubers  only  about '10  per  cent. 

Such  tubers  are  chiefly  valuable  as  a  source  of  carbo- 
hydrates— though  they  also  contain  a  small  proportion  of 
proteids. 

IY.  Green  Vegetables.  —  Cabbage,  cauliflower,  spinach, 
lettuce,  &c.,  are  useful  additions  to  the  diet,  but  their 
value  as  a  source  of  the  proximate  principle  of  the 

21 


322  HUMAN  PHYSIOLOGY 

food  is  not  great  on  account  of  the  amount  of  water  they 
contain. 

They  are  rich  in  the  potash  salts  of  the  organic  acids,  the 
importance  of  which  has  already  been  discussed  (p.  313). 
The  cellulose  forming  the  walls  of  the  cells  in  the  young 
and  growing  part  of  the  leaves  seems  to  be  capable  of  partial 
digestion  in  the  human  intestine. 

Y.  Fungi. — Mushrooms  and  other  such  fungi  do  not  con- 
stitute a  sufficiently  important  part  of  the  diet  to  require 
attention. 

YI.  Fruits. — These  vary  considerably  in  composition. 
Most  are  rich  in  water  and  carbohydrates,  poor  in  proteids, 
and  contain  practically  no  fat.  Their  great  value  lies  in  the 
amount  of  free  and  combined  organic  acids  they  contain. 
Bananas,  which  contain  about  23  per  cent,  of  soluble  carbo- 
hydrates, may  be  considered  as  a  food  of  some  import- 
ance, while  dried  fruits,  especially  dates  and  figs,  which 
contain  between  4  and  5  per  cent,  of  proteids  and  about 
65  per  cent,  of  carbohydrates,  are  of  considerable  nutritive 
value. 

YII.  Nuts. — These,  unlike  the  fruits,  are  for  the  most  part 
poor  in  water  and  carbohydrates  but  rich  in  proteids  and 
fats.  Chestnuts,  however,  contain  an  abundance  of  carbo- 
hydrates, and  a  smaller  proportion  of  proteids  and  fats. 

Cooking. — Few  of  these  food-stuffs  are  used  by  civilised 
man  in  a  raw  and  unprepared  condition.  With  the  object 
of  rendering  them  more  palatable  and  more  easily  digested, 
and  also  in  order  to  destroy  bacteria,  they  are  usually  cooked. 
This  process  of  cooking  produces  important  changes  in  many 
of  the  foods,  and  its  effects  must  be  briefly  considered. 

Milk  and  its  products  are  practically  unaltered  by  cooking. 

Flesh  is  cooked  either  by  exposing  it  directly  to  heat,  or 
by  treating  it  with  boiling  water. 

Roasting,  grilling,  broiling,  and  frying  are  modifications  of 
the  former  method,  and  in  all  of  them  the  heat  at  once 
coagulates  the  proteids  at  the  outer  part  of  the  piece  of  flesh, 
and  thus  forms  a  more  or  less  impermeable  covering,  which 
prevents  the  escape  of  the  juices  of  the  meat.  Hence  these 
methods  of  cooking,  although  bringing  about  a  burning  of 


THE   FOOD  AND  DIGESTION  323 

the  outer  layer  of  the  flesh,  leave  all  the  constituents  in  only 
slightly  altered  proportions. 

On  the  other  hand,  if  a  piece  of  flesh  be  put  into  cold 
water  and  boiled,  the  proteids,  the  salts,  and  the  various 
extractive  bodies  which  give  it  its  flavour  are  extracted,  and, 
as  the  water  warms,  the  fats  also  are  dissolved  out,  and  the 
meat  becomes  poorer  in  these  constituents,  while  the  sur- 
rounding water  becomes  a  soup.  In  this  soup  the  dissolved 
proteids  precipitate  as  the  temperature  rises,  and  when  the 
soup  is  cooled  they  rise  to  the  top  with  the  fats,  and  are 
generally  removed  as  a  scum.  Hence  soups  are  poor  in  the 
proximate  principles  of  food,  but  rich  in  the  extractives  and 
salts  of  meat. 

If,  however,  the  piece  of  meat  to  be  cooked  is  plunged  into 
a  large  quantity  of  boiling  water,  the  proteids  at  its  outer 
part  at  once  coagulate  and  form  a  covering  which  prevents 
the  loss  of  the  nutrient  material  which  occurs  when  the  meat 
is  slowly  boiled. 

Stewing  is  a  modification  of  boiling  by  which  much  of  the 
nutrient  material  of  meat  is  extracted,  but  this  is  served  as  a 
gravy. 

It  is  in  vegetables,  however,  that  cooking  is  of  the  greatest 
importance,  since  by  it  the  cellulose  envelopes  which  enclose 
the  digestible  portions  of  the  plant  are  ruptured. 


II.  DIGESTION 
I.  Structure  of  Alimentary  Canal 

THE  anatomy  and  histology  of  the  alimentary  tract  must 
be  studied  practically.  We  shall  here  merely  give  such  an 
outline  of  the  various  structures  as  will  assist  in  the  com- 
prehension of  their  physiology. 

The  Alimentary  Canal  (Fig.  144)  may  be  divided  into  the 
mouth,  the  oesophagus  or  gullet,  the  stomach,  the  small  and 
large  intestines,  and  three  sets  of  supplementary  structures — 
the  salivary  glands,  the  liver,  and  the  pancreas. 

The  Mouth,  provided  with  its  teeth,  and  surrounded  by 
its  mobile  muscular  wall,  with  the  muscular  tongue  lying  in 
its  floor,  is  the  part  of  the  canal  in  which  the  food  is  broken 
up  and  prepared  for  digestion.  Into  the  mouth  three  pairs 
of  compound  glands — the  Salivary  Glands — open.  The 
parotid,  lined  entirely  by  enzyme -secreting  epithelium, 
opens  on  the  side  of  the  cheek,  while  the  submaxillary 
gland,  composed  partly  of  acini  with  enzyme-secreting,  and 
partly  of  acini  with  mucin-secreting  epithelium,  and  the  sub- 
lingual,  composed  entirely  of  mucin-secreting  acini,  open 
under  the  tongue  (S.C.). 

The  tongue  is  covered  with  a  fine  fur  of  processes,  the  fili- 
form papilla?,  which  are  of  use  in  passing  the  food  backwards 
along  its  surface  in  the  act  of  swallowing.  (For  Organs  of 
Taste  see  p.  137.) 

Posteriorly,  the  mouth  opens  into  the  pharynx  (Ph.)  or 
upper  part  of  the  gullet.  On  each  side,  between  the  mouth 
and  the  pharynx,  is  the  tonsil  (T.),  an  almond-like  mass  of 
lymphoid  tissue.  The  pharynx  is  a  cavity  which  can  be  shut 
off  above  from  the  posterior  nares  by  raising  the  soft  palate, 
and  by  pulling  forward  the  posterior  pharyngeal  wall.  It  is 
surrounded  by  three  constrictor  muscles,  which,  by  contracting 


324 


THE  FOOD   AND  DIGESTION 


325 


from  above  downwards,  force  the  food  down  the  gullet  towards 
the  stomach. 

The  (Esophagus  (Oe.)  is  a  muscular  walled  tube  lined  by  a 
stratified  squamous  epithelium. 
The  muscles,  below  the  lowest 
constrictor  of  the  pharynx,  are 
of  the  visceral  type,  and  are 
arranged  in  two  layers,  an 
outer  longitudinal  layer,  and 
an  inner  circular  layer. 

The  Stomach  is  a  dilatation 
of  the  alimentary  canal  into 
which  the  gullet  opens.  To 
the  left  it  expands  into  a  sac- 
like  cardiac  end  (C.),  and  to 
the  right  it  narrows,  forming 
the  pyloric  end  (Py.).  Like 
the  gullet,  it  is  surrounded 
by  visceral  muscular  fibres, 
arranged  essentially  in  two 
sets.  At  the  cardiac  orifice, 
the  circular  fibres  form  a  not 
very  marked  cardiac  sphincter, 
and  at  the  pyloric  end  they 
form  a  very  thick  and  strong 
pyloric  sphincter. 

The      mucous      membrane, 

Which  is    Covered   by  a   Colum-      FIG    144 -Diagram    of    the   Parts   of 

•/  the  Alimentary  Canal,  from  Mouth 

nar  epithelium,  is  largely  com- 
posed of  tubular  glands,  those 
at  the  cardiac  end  containing 
two  kinds  of  cells,  the  peptic 
and  the  oxyntic  cells,  those 
at  the  pyloric  end  containing 
peptic  cells  alone. 

The  Small  Intestine  is  a  tube  of  about  7  metres  in  length. 
It  has  a  double  muscular  coat  like  the  stomach.  The  mucous 
membrane,  which  is  covered  by  a  columnar  epithelium,  is 
thickly  set  with  simple  test-tube  like  glands — Lieberkuhn's 
follicles — and  is  projected  into  the  lumen  of  the  tube,  as  a 


to  Anus.  T.,  Tonsils;  Ph., 
Pharynx;  S.C.,  Salivary  Glands; 
Oe.,  (Esophagus;  C.,  Cardiac; 
Py-,  Pyloric  Portion  of  Stomach  ; 
D. ,  Duodenum  ;  Li. ,  Liver ;  P. , 
Pancreas  ;  J. ,  Jejunum;  /.,  Ileum  ; 
V.,  Vermiform  Appendix;  Col., 
Colon  ;  It.,  Rectum. 


326  HUMAN  PHYSIOLOGY 

series  of  delicate  finger-like  processes,  the  villi.  The  tissue 
of  the  villi  and  that  between  the  Lieberkiihn's  follicles  is 
chiefly  lymphoid,  and  in  certain  places  this  lymphoid  tissue 
is  massed  in  nodules  which  are  either  placed  singly  or  grouped 
together  in  the  lower  part  of  the  small  intestine  to  form 
Peyer's  patches.  In  the  first  part  of  the  small  intestine — 
the  upper  part  of  the  duodenum  (D.) — the  submucous  layer 
is  full  of  small  branching  glands  lined  by  an  enzyme-secreting 
epithelium  (Brunner's  glands). 

The  Large  Intestine  is  about  2  metres  in  length.  The 
small  intestine  enters  it  at  one  side,  and  the  opening  is 
guarded  by  a  fold  of  mucous  membrane  which  forms  the 
ileo-csecal  valve.  Above  the  opening  of  the  small  intestine 
a  csecal  pouch  exists,  and  at  the  top  of  this  is  the  vermiform 
appendix  ( F.),  a  narrow  tube  with  an  abundance  of  lymph 
tissue  in  its  wall.  Below  the  opening  of  the  small  intestine 
is  the  colon  (Col.\  which,  after  passing  up  the  right  side 
across  the  abdomen  and  down  the  left  side,  takes  an  S-like 
bend  to  end  in  the  rectum  (R.),  which,  passing  forward, 
suddenly  turns  down  and  opens  at  the  anus.  The  sudden 
bend  is  of  importance  in  retaining  the  contents  of  the  rec- 
tum. The  last  part  of  the  rectum  is  surrounded  by  a  strong 
band  of  muscle — the  internal  sphincter  ani — by  which  it 
is  compressed.  The  whole  large  intestine  is  covered  by 
columnar  epithelium,  and  is  studded  with  Lieberkiihn's 
follicles,  in  which  the  epithelium  is  chiefly  mucus-secreting 
in  type.  There  are  no  villi.  The  muscular  coat  of  the  colon 
differs  from  that  of  the  rest  of  the  alimentary  canal,  in  that 
the  longitudinal  fibres  are  arranged  in  three  bands. 

Into  the  duodenum,  the  bile  duct  and  the  duct  of  the 
pancreas  open.  The  bile  duct  is  formed  by  the  union  of 
the  ducts  from  the  lobes  of  the  liver.  Upon  its  course  is  a 
diver ticulum,  the  gall  bladder.  The  Liver  (Li.)  is  a  large 
solid-looking  organ,  formed  originally  as  a  double  outgrowth 
from  the  alimentary  canal.  These  outgrowths  branch,  and 
again  branch,  and  between  them  the  blood  coming  from  the 
mother  to  the  foetus  flows  in  a  number  of  capillary  channels. 
Later,  when  the  alimentary  canal  has  developed,  the  blood 
from  it  is  streamed  between  the  liver  tubules.  In  man  and 
other  mammals,  the  fibrous  tissue  supporting  the  liver  cuts 


THE  FOOD  AND  DIGESTION 


327 


it  up  into  a  number  of  small  divisions,  the  lobules,  each 
lobule  being  composed  of  a  series  of  tubules  arranged  radially 
with  blood  vessels  coursing  between  them. 

The  portal  vein  which  takes  blood  from  the  stomach,  intes- 
tine, pancreas,  and  spleen  breaks  up  in  the  liver  (Fig.  105, 
p.  209),  and  carries  the  blood  between  the  lobules.  From  the 
interlobular  branches,  capillaries  run  inwards  and  enter  a 
central  vein  which  carries  the  blood  from  each  lobule,  and 
pours  it  into  the  hepatic  veins  which  join  the  inferior  vena 
cava.  The  supporting  tissue  of  the  liver  is  supplied  by  the 
hepatic  artery, -and  the  ter- 
minal branches  have  a  very 
free  communication  with 
those  of  the  portal  vein. 

The  Pancreas  is  essenti- 
ally the  same  in  structure  as 
the  parotid  gland.  But  in 
the  lobules  are  certain  little 
masses  of  epithelium  -  like 
cells  closely  packed  to- 
gether, the  Islets  of  Lan- 
gerhans  (Fig.  145). 

The  Nerve  Supply  of  the 
alimentary  canal.  The 
muscles  round  the  mouth 
are  supplied  by  the  fifth, 
seventh,  and  twelfth  cranial 

nerves.       The    nerve    Supply     FiG.145. -Section  of  Pancreas  to  show  Acini 

-ri          of  Secreting  Cells ;  a  large  duct ;  and  m 
Ot    the   Salivary   glands    Will          the  centre  an  Island  of  Langerhans. 

be   considered    later.      The 

pharyngeal   muscles  are  supplied  by  the  ninth  and  tenth 

cranial  nerves,  and  the  oesophagus  is  supplied  by  the  tenth. 

The  stomach  and  intestine  get  their  nerve  fibres  from  two 
sources  (Fig.  126,  p.  263) — above  the  descending  colon  from  the 
vagus  and  the  abdominal  sympathetic,  and  below  this  from 
the  nervi  erigentes  and  abdominal  sympathetic — the  various 
fibres  passing  through  the  abdominal  sympathetic  ganglia. 
In  the  wall  of  the  stomach  and  intestine,  these  nerves  end  by 
forming  an  interlacing  set  of  fibres,  with  nerve  cells  upon 
them,  from  which  fibres  pass  to  the  muscles  and  glands. 


328  ,       HUMAN  PHYSIOLOGY 

One  of  these  plexuses  (Auerbach's)  is  placed  between  the 
muscular  coats — the  other  (Meissner's)  is  placed  in  the 
submucosa. 

II.  Physiology. 
I.  Digestion  in  the  Mouth. 

A.  Mastication. — In  the  inouth,  by  the  act  of  chewing, 
the  food  is  thoroughly  broken  up  and  mixed  with  saliva. 

The  muscular  mechanism  of  mastication  may  be  here 
briefly  indicated. 

MOVEMENTS  OF  MASTICATION. — Movements  of  Lower  Jaw 
in — 1.  Vertical  Plane,  a.  Elevation,  a.  Temporal.  @. 
Masseter.  7.  Internal  Pterygoid.  b.  Depression,  a.  Weight 
of  Jaw.  $.  Anterior  Belly  of  Digastric.  7.  Mylo-  and 
Genio-Hyoid.  B.  Platysma.  Hyoid  fixed  by — Omo-  and 
Sternohyoid,  Sternothyroid,  and  Thyrohyoid.  2.  Horizontal 
Plane — Forwards:  External  Pterygoids.  Backwards:  Pos- 
terior Fibres  of  Temporal.  To  Right:  Left  External  Ptery- 
goid. Right  Temporal  (Posterior  Fibres).  Other  Muscles  of 
Mastication — Buccinator  and  Orbicularis  oris. 

B.  Saliva. — The  saliva  is  formed  by  the  salivary  glands 
(viz.,   the    parotid,    submaxillary,   sublingual,    and    various 
small  glands  in  the  mucous  membrane  of  the  mouth). 

Characters. — It  is  a  somewhat  turbid  frothy  fluid  which, 
when  allowed  to  stand,  throws  down  a  white  deposit  consist- 
ing of  shed  epithelial  scales  from  the  mouth,  leucocytes, 
amorphous  phosphates  of  lime  and  magnesia,  and  generally 
numerous  bacteria.  Its  specific  gravity  is  low — generally 
about  1003.  In  reaction  it  is  neutral  or  faintly  alkaline. 

Chemically  it  is  found  to  contain  a  very  small  proportion 
of  solids.  The  saliva  from  the  parotid  gland  contains  only 
about  0'4  per  cent.,  while  that  from  the  sublingual  may 
contain  from  2  to  3  per  cent.  The  sublingual  and  sub- 
maxillary  saliva  in  man  is  viscous,  from  the  presence  of  mucin 
formed  in  these  glands,  while  the  parotid  saliva  is  free  from 
mucin.  In  addition  to  mucin,  traces  of  proteids  are  readily 
demonstrated,  and  with  these  proteids  is  associated  the  active 
constituent  or  enzyme  of  the  saliva — ptyalin. 

Saliva  generally  contains  traces  of  sulphocyanide  of  potas- 


THE  FOOD  AND  DIGESTION  329 

slum,   and  in   some   pathological    conditions   this   may   be 
markedly  increased. 

The  functions  of  the  saliva  are  twofold  : — 

1.  Mechanical,  to  moisten  the  mouth  and  gullet,  and  thus 
to  assist  in  speaking,  chewing  and  swallowing.     Since  the 
salivary  glands  are  absent  from  aquatic  mammals,  it  would 
appear  that  this  is  the  more  important  function. 

2.  Chemical — Under  the   action   of  the   ptyalin   of  the 
saliva,  polysaccharids,  like  the  starches,  are  broken  down  into 
sugars.     Like  other  enzyme  actions  the  process  requires  the 
presence   of  water  and   a   suitable   temperature,   and  it  is 
stopped  by  the  presence  of  strong  acids  or  alkalies,  by  various 
chemical  substances,  and  by  a  temperature  of  over  60°  C., 
while  it  is  temporarily  inhibited  by  reducing  the  temperature 
to  near  the  freezing  point.     During  the  short  time  the  saliva 
acts  on  the  food  the  conversion  is  by  no  means  complete. 
The  starch  is  first  changed  into  the  dextrins,  giving  a  brown 
colour  with  iodine,  and  hence  called  erythrodextrins,  then  into 
dextrins  which  give  no  colour  with  iodine,  achroodextrins, 
and  lastly  into  the  disaccharid  maltose  (see  p.  317).    (Chemical 
Physiology,  p.  18.) 

Physiology  of  Salivary  Secretion. — In  order  to  study 
the  physiology  of  salivary  secretion,  a  canula  may  be  inserted 
into  the  duct  of  any  of  the  salivary  glands  and  the  flow  of 
saliva  or  pressure  of  secretion  may  be  thus  measured.  In  this 
way  it  may  be  shown  that  the  taking  of  food,  or  simply  the 
act  of  chewing,  and  in  some  cases  the  mere  sight  of  food, 
causes  a  flow  of  saliva.  This  shows  that  the  process  of 
secretion  is  presided  over  by  the  central  nervous  system. 

The  submaxillary  and  sublingual  are  supplied — (1)  By 
branches  from  the  lingual  division  of  the  fifth  cranial  nerve ; 
and  (2)  by  branches  of  the  perivascular  sympathetic  fibres 
coming  from  the  superior  cervical  ganglion.  The  parotid 
gland  is  supplied  by  the  auriculo-temporal  division  of  the 
fifth  and  also  by  sympathetic  fibres  (Fig.  146). 

The  influence  of  these  nerves  has  been  chiefly  studied  on 
the  submaxillary  and  sublingual  glands. 

It  has  been  found  that  when  the  lingual  nerve  is  cut  the 
reflex  secretion  of  saliva  still  takes  place,  but  that,  when  the 
chorda  tympani  (Ch.  T.),  a  branch  from  the  seventh  nerve 


330  HUMAN   PHYSIOLOGY 

which  joins  the  lingual,  is  cut,  the  reflex  secretion  does  not 
occur.  Stimulation  of  the  chorda  tympani  causes  a  copious 
flow  of  watery  saliva,  and  a  dilatation  of  the  blood  vessels  of 
the  glands.  If  atropine  has  been  first  administered  the 
dilatation  of  the  vessels  occurs  without  the  flow  of  saliva, 
indicating  that  the  two  processes  are  independent  of  one 
another.  The  secreting  fibres  all  undergo  interruption  before 


FIG.  146.— Nervous  Supply  of  the  Salivary  Glands.  Par.,  Parotid,  and  S.M.  and 
S.L.,  the  Submaxillary  and  Sublingual  Glands;  VII. ,  the  Seventh  Cranial 
Nerve,  with  Ch.  T.,  the  Chorda  Tympani  Nerve,  passing  to  L.t  the  Lingual 
Branch  of  V.,  the  Fifth  Nerve,  to  supply  the  Glands  below  the  Tongue,  T.  ; 
IX.,  the  Glossopharnygeal  giving  off  J.N.,  Jacobson's  Nerve,  to  the  O., 
Otic  Ganglion,  to  supply  the  Parotid  Gland  through  Aur.  T.,  the  Auriculo- 
temporal  Nerve. 

the  glands  are  reached;  the  fibres  to  the  sublingual  gland 
having  their  cell  station  in  the  submaxillary  ganglion  (S.M.G.), 
the  fibres  to  the  submaxillary  gland  having  theirs  in  a  little 
ganglion  at  the  hilus  of  the  gland  (S.M.).  This  was  demon- 
strated by  painting  the  two  ganglia  with  nicotine.  When 
applied  to  the  submaxillary  ganglion  it  does  not  interfere 
with  the  passage  of  impulses  to  the  submaxillary  gland,  but 
stops  those  going  to  the  sublingual. 

If,  when  the  chorda  tympani  is  stimulated,  the  duct  of  the 


THE  FOOD  AND  DIGESTION  331 

gland  is  connected  with  a  mercurial  manometer,  it  is  found 
that  the  pressure  of  secretion  may  exceed  the  blood  pressure 
in  the  carotid. 

When  the  perivascular  sympathetics,  or  when  the  sym- 
pathetic cord  of  the  neck  is  stimulated,  the  blood  vessels  of 
the  gland  constrict,  and  a  flow  of  very  viscous  saliva  takes 
place. 

On  the  parotid  gland  the  auriculo-temporal  nerve  (Aur.  T.) 
acts  in  the  same  way  as  the  chorda  tympani  acts  on  the 
other  salivary  glands.  But  stimulation  of  the  fifth  nerve 
above  the  otic  ganglion,  from  which  the  auriculo-temporal 
takes  origin,  fails  to  produce  any  effect.  On  the  other  hand, 
stimulation  of  the  glossopharyngeal  nerve  (IX.)  as  it  comes 
off  from  the  brain,  acts  upon  the  parotid  gland,  and  since  the 
glossopharyngeal  is  united  to  the  small  superficial  petrosal 
which  passes  to  the  otic  ganglion,  by  Jacobson's  nerve  (J.N.), 
it  is  obvious  that  these  parotid  fibres  take  this  somewhat 
roundabout  course.  When  the  sympathetic  fibres  to  the 
gland  alone  are  stimulated,  constriction  of  the  blood  vessels 
but  no  flow  of  saliva  occurs,  but  if,  when  the  flow  of  watery 
saliva  is  being  produced  by  stimulating  the  glossopharyngeal 
or  Jacobson's  nerve,  the  sympathetic  fibres  are  stimulated, 
the  amount  of  organic  solids  in  the  parotid  saliva  is  very 
markedly  increased. 

The  nerve  fibres  passing  to  the  salivary  glands  are  pre- 
sided over  by  a  centre  in  the  medulla  oblongata  which  acts 
reflexly.  So  long  as  this  is  intact,  stimulation  of  the  lingual 
or  glossopharyngeal  leads  to  a  reflex  flow  of  saliva.  Other 
nerves  may  also  act  on  this  centre.  Thus,  gastric  irritation, 
when  it  produces  vomiting,  causes  a  reflex  stimulation  of 
salivary  secretion. 

II.  Swallowing. 

The  food  after  being  masticated  is  collected  on  the  surface 
of  the  tongue  by  the  voluntary  action  of  the  buccinators  and 
other  muscles,  and  then,  the  point  of  the  tongue  being 
pressed  against  the  hard  palate  behind  the  teeth,  by  a 
contraction  passing  from  before  backwards,  the  bolus  of 
food  is  driven  to  the  back  of  the  tongue.  When  this  is 


332 


HUMAN   PHYSIOLOGY 


.  TIE7H 


reached  the  act  becomes  involuntary  and  reflex,  and  the 
food  is  forced  through  the  pillars  of  the  fauces  into  the 
pharynx.  It  is  prevented  from  passing  up  into  the  posterior 
nares  by  the  contraction  of  the  palato-pharyngeus  muscle, 
and  of  the  levator  palati.  The  larynx  as  a  whole  is  pulled 
upwards  by  the  stylo-hyoid  and  stylo-thyroid  and  the  thyro- 

hyoid,  and  its  entrance 
into  the  larynx  is  pre- 
vented by  the  closure  of 
upper  part  of  aperture. 
The  arytenoid  carti- 
lages are  pulled  forward 
by  the  thyro-arytenoid 
muscles,  and  approxi- 
mated by  the  aryte- 
noidei,  while  a  cushion 
on  the  posterior  surface 
of  the  epiglottis  be- 
comes applied  to  their 
tips,  forming  a  tri- 
radiate  fissure  or  chink 
through  which  food 
cannot  pass.  The  lateral 
crico  -  arytenoids  also 
approximate  the  vocal 
cords,  and  close  the 
glottis  (Fig.  147). 

The  constrictors  of 
the  pharynx  contract 
from  above  downwards, 
and  force  the  food  into  the  grasp  of  the  oesophagus,  and  this 
by  a  slow  peristaltic  contraction  sends  the  food  onwards  to 
the  stomach.  Under  normal  conditions  this  peristalsis  is  not 
essential.  It  is  abolished  by  section  of  the  vagi.  In  swallow- 
ing liquids,  the  peristalsis  of  the  oesophagus  is  not  brought 
into  play,  but  the  fluid  is  forced  by  the  tongue  down  the 
relaxed  oesophagus  into  the  stomach.  The  passage  of  the 
food  down  the  gullet  may  be  heard  by  applying  a  stetho- 
scope to  the  right  side  of  the  spinal  column,  and  any  delay 
caused  by  a  stricture  may  thus  be  determined. 


FIG.  147.— Vertical  Mesial  Section  of  Mouth  and 
Pharynx  to  show  how,  in  swallowing,  the  food 
slips  along  the  back  of  the  Epiglottis. 


THE   FOOD  AND  DIGESTION  333 

In  swallowing  fluids  two  sounds  are  heard,  one  immediately, 
and  one  after  about  six  seconds. 


III.  Digestion  in  the  Stomach. 

Various  opportunities  have  occurred  and  have  been  taken 
advantage  of  to  study  the  interior  of  the  human  stomach 
during  life.  The  best  known  investigation  of  the  kind  was 
undertaken  by  Dr.  Beaumont,  a  Canadian  physician,  on  the 
person  of  St.  Martin,  a  backwoodsman,  who  had  received  a 
gunshot  wound  in  the  abdomen,  which  had  left  him  with 
an  opening  through  the  front  wall  of  his  stomach.  Dr. 
Beaumont  engaged  St.  Martin  as  his  servant,  and  made  a 
prolonged  and  valuable  study  of  the  changes  which  take 
place  in  the  viscus. 

He  found  that  the  condition  varies  greatly  in  fasting  and 
after  feeding. 

A.  Stomach  during  Fasting. 

The  organ  is  collapsed,  and  the  mucous  membrane  is 
thrown  into  large  ridges.  It  is  pale  in  colour  because  the 
blood  vessels  are  not  dilated.  Movements  are  not  marked 
and  the  secretion  is  scanty,  only  a  little  mucus  being  formed 
on  the  surface  of  the  lining  membrane. 

B.  Stomach  after  Feeding. 

When  food  is  taken,  the  blood  vessels  dilate,  a  secretion  is 
poured  out,  and  movements  of  the  organ  become  more 
marked. 

1.  Vascular     Changes. — The    arterioles    dilate,    and    the 
mucous  membrane  becomes  bright  red  in  colour.      This  is 
a  reflex  vaso-dilator  effect,  impulses,  passing  up  the  vagus 
to  the  vaso-dilator  centre  in  the  medulla,  and  coming  down 
the  vagus  from  that  centre.     Section  of  the  vagi  prevents 
its  onset. 

2.  Secretion. — (a)     Characters    of    Gastric    Juice. — Very 
rapidly  a  free  flow  of  gastric  juice  occurs  from  all  the  glands 


334  HUMAN  PHYSIOLOGY 

in  the  mucous  membrane.  The  gastric  juice  is  a  clear 
watery  fluid,  which  is  markedly  acid  from  the  presence  of 
free  hydrochloric  acid.  In  the  dog  the  free  acid  may 
amount  to  O2  per  cent.,  but  in  man  it  is  less  abundant, 
and  when  the  gastric  juice  is  mixed  with  food  the  acid 
rapidly  combines  with  alkalies  and  with  proteids,  and  is  no 
longer  free.  In  addition  to  the  HC1  small  quantities  of 
organic  salts  are  present.  Traces  of  proteids  may  also  be 
demonstrated,  and  with  these  two  enzymes  are  associated- 
one  a  proteolytic  or  proteid-digesting  enzyme,  pepsin,  the 
other  a  milk-curdling  enzyme,  rennin. 

(b)  Course  of  Gastric  Digestion — (1)  Amylolytic  Period.— 
The  action  of  the  gastric  juice  does  not  at  once  become 
manifest.  For  half-an-hour  after  the  food  is  swallowed  the 
ptyalin  of  the  saliva  goes  on  acting,  and  the  various  micro- 
organisms swallowed  with  the  food  grow  and  multiply,  and 
thus  there  is  a  continuance  of  the  conversion  of  starch  to 
sugar  which  was  started  in  the  mouth,  and  at  the  same  time 
the  micro-organisms  go  on  splitting  the  sugar  to  form  lactic 
acid,  which  may  thus  be  regarded  as  a  normal  constituent 
of  the  stomach  during  the  first  half-hour  after  a  mixed 
meal. 

(2)  Proteolytic  Period. — Before  the  amylolytic  period  is 
completed,  the  gastric  juice  has  begun  its  special  action  on 
proteids.  This  may  be  readily  studied  by  placing  some 
coagulated  proteid  in  gastric  juice,  or  in  an  extract  of  the 
mucous  membrane  of  the  stomach  made  with  dilute  hydro- 
chloric acid,  and  keeping  it  at  the  temperature  of  the  body. 
The  proteid  swells,  becomes  transparent,  and  dissolves.  The 
solution  is  coagulated  on  boiling — a  soluble  native  proteid 
has  been  formed.  Very  soon  it  is  found  that,  if  the  soluble 
native  proteid  is  filtered  off,  the  filtrate  gives  a  precipitate 
on  neutralising,  showing  that  an  acid  proteate  has  been 
produced.  If  the  action  is  allowed  to  continue  and  the  acid 
proteate  precipitated  and  filtered  off,  it  will  be  found  that 
the  filtrate  gives  a  precipitate  on  saturating  with  common 
salt,  showing  that  a  proto-proteose  has  been  formed.  Along 
with  this  a  certain  amount  of  hetero-proteose  is  also  formed. 


THE  FOOD  AND   DIGESTION  335 

It  is  characterised  by  being  precipitated  on  neutralisation 
and  by  being  insoluble  in  distilled  water.  On  filtering  off 
these,  the  filtrate  yields  a  precipitate  on  saturating  with 
sulphate  of  ammonia,  indicating  the  formation  of  a  deutero- 
proteose,  and,  if  the  filtrate  from  this  be  tested,  the  presence 
of  a  proteid  may  be  demonstrated.  Peptone  has  been 
produced.  (Chemical  Physiology,  p.  18.) 

These    changes    may  be    represented    in    the    following 
table  :— 

Coagulated  Proteid. 

Soluble  Native  Proteid. 
Acid  Proteate. 


|  I 

Proto-proteose.      Hetero-proteose. 

Deutero-proteose.      Deutero-proteose. 

Peptone.  Peptone. 

The  process  is  one  of  breaking  down  a  complex  molecule 
into  simpler  molecules,  probably  with  hydration. 

The  object  of  this  was  formerly  supposed  to  be  to  allow  of 
the  diffusion  of  the  proteid  in  the  form  of  peptone  through 
the  wall  of  the  intestine.  It  is  now  known  that  absorption 
is  not  due  to  diffusion,  and  it  is  more  probable  that  the 
change  to  the  simplest  proteid  molecule  is  a  necessary  step 
in  the  building  up  of  the  proteid  into  the  special  protoplasm 
of  the  body  of  the  particular  animal. 

On  certain  proteids  and  their  derivatives  the  gastric  juice 
has  a  special  action.  On  collagen  the  HC1  acts  slightly  in 
converting  it  to  gelatin.  On  gelatin  the  gastric  juice  acts, 
converting  it  to  a  gelatin  peptone. 

On  nucleo-proteids  it  acts  by  digesting  the  proteid  part 
and  leaving  the  muclein  undissolved. 

Haemoglobin  is  broken  down  into  hsematin  and  globin,  and 
the  latter  is  changed  into  peptone. 

The  casein  of  milk  is  first  coagulated  and  then  changed  to 
peptone.  The  coagulation  is  brought  about  by  the  presence 
of  the  second  enzyme  of  the  gastric  juice — rennin. 


336  HUMAN  PHYSIOLOGY 

This  may  be  separated  from  pepsin  in  various  ways,  and 
unlike  pepsin  it  acts  in  a  neutral  medium. 

The  change  set  up  by  it  seems  to  be  due  to  a  splitting 
of  the  soluble  lime  salt  of  casein  which  exists  in  milk 
into  calcic  paracasein,  which  is  insoluble  and  is  thrown 
down,  while  a  small  quantity  of  whey  albumin  remains 
in  solution.  The  nuclein  part  of  the  paracasein  remains 
undigested. 

On  Fats  the  gastric  juice  has  no  action,  but,  when  these 
are  contained  in  the  protoplasm  of  cells,  it  sets  them  free  by 
digesting  the  proteid  covering. 

On  Carbohydrates  the  free  mineral  acid  of  the  gastric 
juice  has  a  slight  action  at  the  body  temperature,  splitting 
the  polysaccharids  and  disaccharids  into  monosaccharids. 

(c)  Digestion  of  the  Stomach  Wall.— When  the  wall  of 
the  stomach  dies  either  in  whole,  as  after  the  death  of  the 
animal,  or  in  part,  as  when  an  artery  is  occluded  or  ligatured, 
the  dead  part  is  digested  by  the  gastric  juice  and  the  wall  of 
the  stomach  may  be  perforated. 

(d)  Antiseptic  Action  of  the  Gastric  Juice. — In  virtue  of 
the  presence  of  free  HC1  the  gastric  juice  has  a  marked  action 
in  inhibiting  the  growth  of  or  in  killing  bacteria.     The  bacillus 
of  cholera  is  peculiarly  susceptible,  and  a  healthy  condition  of 
the  stomach  is  thus  a  great  safeguard  against  the  disease. 
Other  organisms,  while  they  do  not  multiply  in  the  stomach, 
pass  on  alive  to  the  intestine  where  they  may  again  become 
active.     When  HC1  is  not  formed  in  sufficient  quantities  to 
exist  free  in  the   stomach,   the   activity   of  these  bacteria 
may  lead   to   various  decompositions  and  to  many  of  the 
symptoms  of  dyspepsia. 

(e)  Source  of  the  Constituents  of  the   Gastric  Juice.— 

The  hydrochloric  acid  is  formed  at  the  cardiac  end  of  the 
stomach.  This  may  be  shown  by  isolating  a  part  of  the 
stomach  so  that  it  opens  on  the  surface.  Since  the  parietal 
or  oxyntic  cells  are  confined  to  this  portion  of  the  stomach,  it 
may  be  concluded  that  they  are  the  producers  of  the  acid. 
They  manufacture  it  from  the  NaCl  of  the  blood  plasma. 


THE  FOOD   AND  DIGESTION 


337 


Probably  the  C02  liberated  in  the  cells  seizes  on  some  of  the 
Na  and  turns  out  HC1. 

The  Pepsin  and  Rennin  are  produced  in  the  chief  or  peptic 
cells  which  line  the  glands  both  of  the  cardiac  and  pyloric 
parts  of  the  stomach.  During  fasting  granules,  are  seen  to 
accumulate  in  these  cells,  and  when  the  stomach  is  active 
they  are  discharged.  These  granules  are  not  pepsin  but  the 
forerunner  of  pepsin — pepsinogen. 

(/)  Influence  of  Various  Diets  upon  the  Gastric  Juice. — 
This  has  been  chiefly  worked  out  by  Pawlow  on  dogs.  By  a 


FIG.  148. — Diagram  of  Pawlow's  Pouch  made  on  the  Stomach  of  a  Dog. 


longitudinal  incision  along  the  great  curvature  of  the  stomach 
he  separated  a  V-shaped  piece  (Fig.  148).  The  cut  edges  of 
the  stomach  are  then  stitched  up  and  the  sides  of  the 
V-shaped  portion  are  also  united,  except  that  at  the  apex  they 
are  attached  to  the  skin  surface.  The  mucous  membrane 
between  the  stomach  cavity  and  the  "  pouch  "  formed  is  then 
united,  and  a  small  stomach  is  thus  produced,  still  connected 
with  the  nerves  and  vessels,  but  separated  from  the  main 
cavity  and  pouring  its  contents  on  to  the  surface.  Any 
modification  in  the  secretion  of  the  stomach  is  indicated  by 
a  modification  in  the  secretion  from  this  pouch. 

22 


338  HUMAN   PHYSIOLOGY 

It  has  thus  been  found  that — (1)  The  amount  of  secretion 
depends  upon  the  amount  of  food  taken.  (2)  The  amount 
and  course  of  secretion  varies  with  the  kind  of  food  taken. 
Thus,  with  flesh  the  secretion  reaches  its  maximum  at  the  end 
of  one  hour,  persists  for  an  hour  and  then  rapidly  falls,  while 
with  bread  it  reaches  its  maximum  at  the  end  of  one  hour, 
rapidly  falls  but  persists  for  a  much  longer  period  than  in 
the  case  of  flesh.  (3)  The  digestive  activity  of  the  juices 
varies  with  the  kind  of  food  and  with  the  course  of  diges- 
tion. It  is  higher  and  persists  longer  after  a  diet  of  bread, 
which  is  difficult  to  digest,  than  after  a  diet  of  flesh,  which  is 
more  easily  digested.  (4)  The  percentage  of  acid  does  not 
vary  markedly.  When  more  acid  is  required  more  gastric 
juice  is  secreted.  (5)  The  work  done  by  the  gastric  glands  is 
greater  in  the  digestion  of  bread  than  in  the  digestion  of 
flesh. 

(g)  Nervous  Mechanism  of  Gastric  Secretion. — It  has 
been  proved  that  in  the  dog  the  secretion  of  gastric  juice  can 
go  on  after  the  nerves  to  the  stomach  have  been  divided,  and 
this  has  been  ascribed  to  a  reflex  stimulation  of  the  nerve 
plexus  in  the  submucosa.  But  while  this  is  the  case,  the 
vagus  also  exercises  a  direct  influence.  This  was  proved  by 
experiments  on  dogs  in  which  Pawlow's  pouch  had  been 
made,  and  in  which  the  oesophagus  was  opened  in  the 
neck,  so  that  when  food  was  taken  it  did  not  pass  into  the 
stomach.  It  was  found  that  letting  the  dog  swallow  food, 
which  of  course  escaped  by  the  opening  in  the  oesophagus, 
or  merely  showing  food  to  the  dog,  caused  a  secretion  of 
gastric  juice  if  the  vagi  were  intact,  but  not  after  they  were 
divided. 

3.  Movements  of  the  Stomach. — These  have  been  studied 
by  feeding  an  animal  with  food  containing  bismuth,  and  then 
applying  X  rays,  which  are  intercepted  by  the  coating  of 
bismuth,  so  that  a  shadow  picture  of  the  shape  of  the 
stomach  is  given  (Fig.  149). 

It  is  found  that,  soon  after  food  is  taken,  a  constriction 
forms  about  the  middle  of  the  stomach  and  slowly  passes  on 
towards  the  pylorus.  Another  constriction  forms  and  follows 
the  first,  and  thus  the  pyloric  part  of  the  stomach  is  set  into 


THE  FOOD  AND   DIGESTION  339 

active  peristalsis.  The  cardiac  fundus  acts  as  a  reservoir,  and, 
by  a  steady  contraction,  presses  the  gastric  contents  into  the 
more  active  pylorus,  so  that  at  the  end  of  gastric  digestion  it 
is  completely  emptied. 

The  pylorus  is  closed  by  the  strong  sphincter  muscle, 
which,  however,  relaxes  from  time  to  time  during  gastric 
digestion  to  allow  the  escape  of  the  more  fluid  contents  of 
the  stomach  into  the  intestine.  These  openings  are  at  first 
slight  and  transitory,  but  as  time  goes  on  they  become  more 
marked  and  more  frequent,  and  when  gastric  digestion  is 
complete — usually  at  the  end  of  five  or  six  hours — the 


FIG.  149. — Tracings  of  the  shadows  of  the  contents  of  the  stomach  and  intestine  of  a 
cat  two  hours  after  feeding  (A)  with  boiled  lean  beef,  and  (B)  with  boiled  rice. 
The  small  divisions  of  the  food  in  some  of  the  intestinal  loops  represent  the 
process  of  rhythmic  segmentation  (see  p.  354).  (CANNON.) 

sphincter  is  completely  relaxed  and  allows  the  stomach 
to  be  emptied.  The  rate  of  passage  from  the  stomach  of 
various  kinds  of  food — proteid,  carbohydrate,  and  fat — has 
been  studied  by  feeding  cats  with  equal  amounts  of  each 
special  food  mixed  with  bismuth,  and  then,  by  X  rays, 
getting  the  outline  of  the  contents  of  the  small  intestine 
at  different  periods.  Carbohydrates  were  found  toipass  on 
most  rapidly  and  fats  most  slowly. 

Nervous  Mechanism  of  the  Movements. — Even  after  the 
section  of  all  the  gastric  nerves,  movements  of  the  stomach 
may  be  observed,  but  the  mechanism  of  these  has  not  been 
fully  studied.  The  action  of  the  vagus  and  sympathetic 
fibres  is  complicated,  and  their  influence  on  the  wall  of  the 
stomach  and  the  sphincters  requires  further  investigation. 


340  HUMAN  PHYSIOLOGY 

Speaking  generally,  the  vagus  seems  to  increase  the  move- 
ments, while  the  sympathetic  fibres  check  them ;  but  the 
vagus  seems  to  inhibit  the  cardiac  sphincter. 


Absorption  from  the  Stomach. 

The  stomach  appears  to  play  a  small  part  in  the  absorp- 
tion of  food.  Very  little  water  is  absorbed,  fats  are  not 
absorbed,  but  sugar  and  peptones  seem  to  be  absorbed  to  a 
considerable  extent. 


Importance  of  the  Stomach  in  Digestion. 

The  chief  function  of  the  stomach  is  as  a  reservoir  for  the 
food.  While  it  plays  a  certain  part  in  digestion  its  action 
is  by  no  means  indispensable,  for  it  has  been  removed  in 
animals  and  in  men  without  disturbance  of  the  health.  Pro- 
bably the  antiseptic  action  of  its  secretion  is  of  very  consider- 
able practical  importance. 

Vomiting. 

Sometimes  the  stomach  is  emptied  upwards  through  the 
gullet  instead  of  downwards  through  the  pylorus.  This  act 
of  vomiting  is  generally  a  reflex  one,  resulting  from  irritation 
of  the  gastric  mucous  membrane  and  more  rarely  from 
stimulation  of  other  nerves.  Usually  the  act  is  preceded  by 
a  feeling  of  nausea  and  by  a  free  secretion  of  saliva.  In 
vomiting,  the  glottis  is  closed,  and,  after  a  forced  inspiratory 
effort  by  which  air  is  drawn  down  into  the  gullet,  a  forced 
and  spasmodic  expiration  presses  on  the  stomach,  while  at 
the  same  time  the  cardiac  sphincter  is  relaxed  through  the 
action  of  the  vagus,  and  the  contents  of  the  stomach  are 
sent  upwards.  They  are  at  first  prevented  from  passing  into 
the  nares  by  the  contraction  of  the  soft  palate ;  but,  as  the 
act  continues,  these  muscles  are  overcome,  and  the  vomited 
matter  escapes  through  mouth  and  nose.  The  wall  of  the 
stomach  also  seems  to  act,  but  its  action  is  non-essential, 
since  vomiting  may  be  produced  in  an  animal  in  which  a 
bladder  has  been  made  to  replace  the  stomach. 


THE   FOOD   AND   DIGESTION  341 

The  centre  which  presides  over  the  act  is  in  the  medulla 
oblongata,  and  while  it  is  usually  reflexly  called  into  action, 
it  may  be  stimulated  directly  by  such  drugs  as  apomorphine. 

IY.  Intestinal  Digestion 

After  being  subjected  to  gastric  digestion  the  food  is  gene- 
rally reduced  to  a  semi-fluid  grey  pultaceous  condition  of 
strongly  acid  reaction  known  as  chyme,  and  in  this  condition 
it  enters  the  duodenum. 

Here  it  meets  three  different  secretions : — 
Bile. 

Pancreatic  secretion. 
Intestinal  secretion. 

A.  Bile. 

i.  Characters  and  Composition. — The  bile  is  the  secretion 
of  the  liver,  and  it  may  be  procured  for  examination — (a) 
From  the  gall  bladder,  or  (6)  from  the  bile  passages  by 
making  a  fistula  into  them.  Bile  which  has  been  in  the 
gall  bladder  is  richer  in  solids  than  bile  taken  directly  from 
the  ducts,  because  water  is  absorbed  by  the  walls  of  the 
bladder  and  the  bile  thus  becomes  concentrated. 

Analyses  of  gall  bladder  bile  thus  give  no  information  as 
to  the  composition  of  the  bile  when  formed.  In  several  cases, 
where  surgeons  have  produced  biliary  fistulse,  opportunities 
have  occurred  of  procuring  the  bile  directly  from  the  ducts 
during  life  in  man. 

Such  bile  has  a  somewhat  orange-brown  colour,  and  is 
more  or  less  viscous,  but  not  nearly  so  viscous  as  bile  taken 
from  the  gall  bladder.  It  has  a  specific  gravity  of  almost 
1005,  while  gall  bladder  bile  has  a  specific  gravity  of  about 
1030.  Its  reaction  is  slightly  alkaline,  and  it  has  a  charac- 
teristic smell. 

It  contains  about  2  per  cent,  of  solids,  of  which  more  than 
half  are  organic. 

The  most  abundant  solids  are  the  salts  of  the  bile  acids. 
In  man  the  most  important  is  glycocholate  of  soda.  Tauro- 
cholate  of  soda  occurs  in  small  amounts.  These  salts  are 
readily  separated  from  an  alcoholic  solution  of  dried  bile  by 


342  HUMAN   PHYSIOLOGY 

the  addition  of  water-free  ether,  which  makes  them  separate 
out  as  crystals.     (Chemical  Physiology,  p.  21.) 

Glycocholic  acid  splits  into  glycin,  amido-acetic  acid — 

H    0 

H\        1     » 

>N— C— C— O— H 

i 

and  a  body  of  unknown  constitution,  cholalic  acid,  C^H^O^ 
Taurocholic   acid  yields  amido-ethane-sulphuric  acid   or 
taurin.     This  is  a  molecule  closely  resembling  amido-acetic 
acid  linked  to  sulphuric  and  cholalic  acids. 

Since  both  acids  contain  nitrogen  they  must  be  derived 
from  proteids.  That  they  are  formed  in  the  liver  and  not 
merely  excreted  by  it,  is  shown  by  the  fact  that,  while 
they  accumulate  in  the  blood  if  the  bile-duct  is  ligatured, 
they  do  not  appear  if  the  liver  is  excluded  from  the 
circulation. 

Action  of  Bile  Salts. — 1.  The  bile  salts  are  solvents  of  fats 
and  fatty  acids,  and  they  thus  assist  in  the  digestion  and 
absorption  of  fats.  When  bile  is  excluded  from  the  intes- 
tines no  less  than  30  per  cent,  of  the  fats  of  the  food  may 
escape  absorption  and  appear  in  the  faeces.  When  this  is 
the  case  the  feces  have  a  characteristic  white  or  grey  appear- 
ance from  the  abundance  of  fat. 

2.  These  salts  keep  cholesterin  in  solution. 

3.  While  the  salts  have  no  action  on  proteids,  free  tauro- 
cholic  acid  precipitates  native  proteids  and  acid  proteate. 
In  the  human  intestine  this  is  an  action  of  no  importance. 

4.  These  salts  are  powerful  ha^molytic  agents,  and  rapidly 
dissolve  haemoglobin  out  of  the  erythrocytes. 

Bile  Pigments. — These  amount  to  only  about  0*2  per  cent. 
In  human  bile  the  chief  pigment  is  an  orange-brown  sub- 
stance, bilirubin,  C32H36N4O6,  while  in  the  bile  of  herbivora, 
biliverdin,  a  green  pigment  somewhat  more  oxidised  than 
bilirubin,  C^H^N^Og,  is  more  abundant.  By  further  oxi- 
dation with  nitrous  acid  other  pigments — blue,  red,  and 
yellow — are  produced,  and  this  is  used  as  a  test  for  the  pre- 
sence of  bile  pigments  (Gmelin's  test).  (Chemical  Physiology, 
p.  21.) 


THE  FOOD   AND   DIGESTION  343 

The  pigments  are  closely  allied  to  hsematoporphyrin 
and  hsematoidin,  and  they  are  derived  from  haemoglobin. 
Their  amount  is  greatly  increased  when  haemoglobin  is 
set  free  or  injected  into  the  blood.  They  are  formed 
in  the  liver,  since,  when  the  liver  is  excluded  from  the 
circulation,  the  injection  of  haemoglobin  does  not  cause 
their  formation. 

The  liver  has  the  property  of  excreting  not  only  these 
pigments  formed  by  itself,  but  also  other  pigments.  Thus 
the  liver  of  the  dog  can  secrete  the  characteristic  pigment  of 
sheep's  bile. 

Cholesterin  is  a  monatomic  alcohol — C26H43OH — which 
occurs  free  in  small  amounts  in  the  bile.  It  is  very  insoluble 
and  is  kept  in  solution  by  the  salts  of  the  bile  acids.  It 
readily  crystallises  in  rhombic  plates,  generally  with  a  notch 
out  of  the  corner.  On  account  of  its  insolubility,  when  it  is 
in  excess  in  the  bile  or  when  the  bile  salts  are  decreased, 
it  may  form  concretions  or  biliary  calculi — gall  stones — 
which  may  accumulate  in  the  gall  bladder  and  may  get 
caught  in  the  bile  passages,  obstructing  the  flow  of  bile  and 
leading  to  its  absorption  throughout  the  system.  Jaundice 
is  thus  produced.  When  these  stones  are  forced  along  the 
bile  passages  as  a  result  of  muscular  contraction,  intense 
agony — biliary  colic — may  be  produced.  When  they  are 
passed  by  the  rectum,  their  nature  is  readily  demonstrated 
by  breaking  them  up  in  a  mortar,  dissolving  in  hot  alcohol, 
and  allowing  the  solution  to  cool,  when  the  characteristic 
crystals  separate  out.  (Chemical  Physiology,  p.  12.)  The 
source  of  the  cholesterin  of  the  bile  is  not  definitely  known. 
It  is  not  an  excretion  of  cholesterin  formed  elsewhere,  because 
the  injection  of  cholesterin  does  not  lead  to  an  increase  in  the 
amount  in  the  bile.  According  to  Naunyn's  observations 
it  is  most  abundant  in  cases  of  inflammation  of  the  bile 
passages,  and  he  therefore  thinks  it  is  formed  by  the 
breaking  down  of  the  epithelium  lining  these  ducts. 

Fats  and  Lecithin. — The  true  fats  and  the  phosphorus  con- 
taining lecithin  are  present  in  small  amounts  in  the  bile,  and 
apparently  they  are  derived  from  the  fats  of  the  liver  cells; 
and  they  may  be  increased  in  amount  by  the  administration 
of  fatty  food. 


344  HUMAN   PHYSIOLOGY 

Nucleo-proteid  and  Mucin. — The  bile  owes  its  viscosity  to 
the  presence  of  a  mucin-like  body,  which,  however,  does  not 
yield  sugar  on  boiling  with  an  acid  and  which  contains  phos- 
phorus. It  is  precipitated  by  acetic  acid,  but  is  soluble  in 
excess.  It  is  therefore  a  nucleo-proteid.  In  some  animals 
a  certain  amount  of  mucin  is  also  present.  (Chemical 
Physiology,  p.  21.) 

Inorganic  Constituents. — The  most  abundant  salt  is  phos- 
phate of  calcium.  Phosphate  of  iron  is  present  in  traces. 
Carbonate  of  soda  and  of  calcium  and  chloride  of  sodium  are 
the  other  chief  salts. 

2.  Flow  of  Bile. — The  bile,  when  secreted  by  the  liver 
cells,  may  accumulate  in  the  bile  passages  and  gall  bladder 
to  be  expelled  under  the  influence  of  the  contraction  of  the 
muscles  of  the  ducts  or  of  the  pressure  of  the  abdominal 
muscles  upon  the  liver.  The  flow  of  bile  into  the  intestines 
thus  depends  upon — 1st,  The  secretion  of  bile;  2nd,  the 
expulsion  of  bile  from  the  bile  passages.  It  is  exceedingly 
difficult  to  separate  the  action  of  these  two  factors.  The  flow 
of  bile  in  the  human  subject  has  now  been  studied  in  several 
cases  in  which  the  surgeon  has  had  to  make  a  fistula  into 
the  gall  bladder  through  which  all  the  bile  secreted  escaped 
and  could  be  collected. 

The  flow  of  bile  begins  in  intra-uterine  life  before  the 
twelfth  week,  and  it  continues  without  intermission  through- 
out the  whole  of  life,  even  during  very  prolonged  fasts.  The 
taking  of  food  increases  the  flow  of  bile,  and  the  extent  to 
which  it  is  increased  depends  largely  on  the  kind  of  food 
taken.  In  the  dog  a  proteid  meal  has  the  most  marked 
effect,  a  fatty  meal  a  less  marked  effect,  and  a  carbohydrate 
meal  hardly  any  effect.  The  increased  flow  of  bile  following 
the  taking  of  food  does  not  reach  its  maximum  till  six  or 
nine  hours  after  the  food  is  taken,  and  some  observers  have 
found  that  the  period  of  maximum  flow  is  even  further  pro- 
longed. Very  often,  immediately  after  food  is  taken,  there 
is  a  markedly  increased  flow  due  to  the  stimulation  of  the 
muscles  of  the  bile  passages,  but  the  later  increase  seems  to 
be  due  to  a  true  increased  formation  of  bile.  When  the  indi- 
vidual is  taking  a  liberal  diet  the  secretion  of  bile  appears  to 


THE   FOOD   AND   DIGESTION  345 

be  greater  than  when  the  diet  is  low.  In  fever  there  is  a  very 
marked  fall  in  the  secretion,  the  fluid  flowing  from  a  fistula 
becoming  colourless  and  almost  devoid  of  bile  salts  and  pig- 
ments. Certain  drugs  markedly  modify  the  formation  of 
bile — the  salts  of  the  bile  acids  stimulating  the  liver  to  form 
more  solids  and  to  secrete  more  water,  the  salicylates  acting 
in  much  the  same  way,  and  all  drugs  which  cause  haemolysis 
— i.e.  the  solution  of  the  pigment  of  the  erythrocytes — pro- 
ducing an  increased  formation  of  bile  pigments. 

Influence  of  Nerves  upon  the  Flow  of  Bile — (a)  Expulsion 
of  Bile. — There  is  good  evidence  that  fibres  pass  to  the  muscles 
of  the  bile  passages  and  may  cause  an  expulsion  of  bile  by 
stimulating  them  to  contract. 

(b)  Secretion  of  Bile. — There  is  no  evidence  that  nerve 
fibres  act  directly  upon  the  secretion  of  bile.  This 
appears  to  be  governed  by  the  nature  of  the  material 
brought  to  the  liver  by  the  blood  and  by  the  activity  of 
the  liver  cells. 

3.  Mode  of  Formation  of  Bile. — It  has  been  seen  that  the 
bile  salts  and  pigments  are  actually  formed  in  the  liver  cells, 
and  there  is  good  evidence  that  the  water  of  the  bile  is  not 
a  mere  transudation  but  is  the  product  of  the  living  activity 
of  these  cells.     The  pressure  under  which  bile  is  secreted 
may  be  determined  by  fixing  a  canula  in  the  bile  duct  or  in 
a  biliary  fistula.     In  man  the  pressure  is  as  much  as  20  to 
30  mm.  Hg,  while  the  pressure  in  the  portal  vein  of  the  dog 
is  only  7  to  16  mrn.  Hg. 

4.  Nature  of  Bile. — Bile  is  not  a  secretion  of  any  import- 
ance in  digestion.     It  has  no  action  on  proteids  or  carbo- 
hydrates, and  its  actions  on  fats  is  merely  that  of  a  solvent. 
Its  secretion  in  relationship  to  food  does  not  indicate  that  it 
plays  an  active  part  in  digestion.     It  is  formed  during  intra- 
uterine  life  and  during  fasting,  and  it  is  produced  many 
hours  after  food  is  taken,  when  digestive  secretions  are  no 
longer  of  use  in  the  alimentary  canal.     Digestion  can  go  on 
quite  well  without  the  presence   of  bile   in   the   intestine, 
except  that  the  fats  are  not  so  well  absorbed.     The  composi- 
tion of  bile  strongly  suggests  that  it  is  a  waste  product.     The 


346  HUMAN   PHYSIOLOGY 

pigment  is  the  result  of  the  decomposition  of  haemoglobin 
and  the  acids  are  the  result  of  proteid  disintegration. 

All  these  facts  seem  to  indicate  that  bile  is  the  medium 
by  which  the  waste  products  of  hepatic  metabolism  are 
eliminated,  just  as  the  waste  products  of  the  body  generally 
are  eliminated  by  the  kidneys. 

B.  Pancreatic  Secretion. 

The  secretion  of  the  pancreas  may,  in  the  dog,  be  procured 
by  making  either  a  temporary  or  a  permanent  fistula.  In 
the  former  case  the  duct  is  exposed,  and  a  canula  fastened 
in  it ;  in  the  second  the  duct  is  made  to  open  on  the  surface 
of  the  abdomen. 

1.  Characters  and  Composition. — When  obtained  from  a 
temporary  fistula,  immediately  after  the  operation,  the  pan- 
creatic juice  is  a  clear,  slimy  fluid,  with  a  specific  gravity 
often  of  1030   and   an   alkaline   reaction.      It   contains   an 
abundance  of  a  native  proteid  having  the  characters  of  a 
globulin,  and  the  alkalinity  is  probably  due  to  carbonate 
and  alkaline  phosphate  of  soda.     From  a  permanent  fistula 
a  more  abundant  flow  of  more   watery  secretion   may  be 
collected. 

2.  Action. — Closely  associated  with  the  proteids,  and  pre- 
cipitated by  alcohol  along  with  them,  are  the  enzymes  upon 
which  the  action  of  the  pancreatic  juice  depends.     They  are 
three  in  number.     (Chemical  Physiolgy,  p.  20.) 

1st.  A  Proteolytic  Enzyme — Trypsin.  This,  in  a  weakly 
alkaline  or  neutral  fluid,  converts  native  proteids  into 
peptones,  and  is  capable  of  still  further  breaking  up  these 
peptones  into  simpler  non-proteid  bodies.  It  does  not  cause 
solid  proteids  to  swell  up  but  simply  erodes  them  away. 

The  pancreatic  juice  brings  about  this  breaking  down  in 
stages.  Fibrin  and  similar  bodies  first  pass  into  the  condi- 
tion of  soluble  native  proteids  and  then  into  deutero-proteose, 
while  boiled  egg  white  appears  at  once  to  yield  deutero-pro- 
teose.  The  deutero-proteose  is  then  changed  into  peptone, 
and  part  of  that  peptone  may  split  up  still  further  into  a 


THE  FOOD   AND   DIGESTION  347 

series  of  bodies  which  no  longer  give  the  biuret  test.  These 
consist  chiefly  of  the  component  amido  -  acids,  of  which 
the  most  important  are  leucin  and  tyrosin,  and  ammonia 
compounds  (see  p.  11). 

Tryptophan. — If  chlorine  water  is  added  to  a  pancreatic 
digestion,  which  has  proceeded  for  a  long  time,  a  rose-red 
colour  is  struck,  and  the  substance  yielding  this,  to  which 
the  name  of  tryptophan  has  been  given,  appears  to  be  amido- 
acetic  acid  linked  to  skatol  (see  p.  399). 

While  trypsin  has  the  power  of  splitting  some  of  the 
peptone  in  the  manner  indicated,  it  has  probably  little 
opportunity  of  doing  so  in  the  intestine,  because  the  proteid 
is  rapidly  absorbed  as  it  is  changed  to  peptone. 

On  nucleo  -  proteids  the  trypsin  acts  by  digesting  the 
proteid  and  dissolving  the  nucleic  acid  so  that  it  can  be 
absorbed. 

On  collagen  and  elastin  the  trypsin  has  little  action ;  but 
on  gelatin  it  acts  as  upon  proteids. 

2nd.  An  Amylolytic  Enzyme — Amylopsin.  This  acts  in 
the  same  way  as  ptyalin,  but  more  powerfully,  converting  a 
certain  part  of  the  maltose  into  dextrose.  It  acts  best  in  a 
faintly  acid  medium. 

3rd.  A  Fat  Splitting  Enzyme — Pialyn.  This  is  the  most 
easily  destroyed  and  the  most  difficult  to  separate  of  the 
zymins.  It  breaks  the  fats  into  their  component  glycerin  and 
fatty  acids.  The  fatty  acid  links  with  the  alkalies  which  are 
present  to  form  soaps,  and  in  this  form,  or  dissolved  as  free 
fatty  acids  in  the  bile,  they  are  absorbed.  But  the  formation 
of  soaps  also  assists  the  digestion  of  fats  by  reducing  them 
to  a  state  of  finely  divided  particles,  an  emulsion  upon  which 
the  pialyn  can  act  more  freely.  This  process  of  emulsifica- 
tion  is  assisted  by  the  presence  of  proteid  in  the  pancreatic 
juice  and  also  by  the  presence  of  bile. 

That  these  enzymes  are  independent  of  one  another  is 
shown  by  many  facts. 

1.  Amylopsin  does  not  appear  till  a  month  after  birth. 

2.  Amylopsin  is  taken  up  by  dry  glycerin  while  trypsin 
is  not. 

3.  Trypsin  may  be  precipitated  and  separated  by  shaking 
with  collodion. 


348  HUMAN   PHYSIOLOGY 

4.  Trypsin  acts  in  O'Ol  percent,  ammonia  while  amylopsin 
does  not. 

5.  The  proportion  of  the  zymins  varies  with  the  character 
of  the  diet. 

This  is  well  shown  by  experiments  carried  out  in  Paw  low's 
laboratory  upon  dogs  with  pancreatic  fistuke.  The  effects  of 
diets  of  milk,  bread,  and  flesh  were  compared,  in  each  case 
the  amount  of  the  food  given  containing  the  same  amount 
of  nitrogen  (proteid).  The  total  quantity  of  ferment  unit 
is  got  by  multiplying  the  quantity  of  the  juice  in  ccrn.  by 
the  strength  of  the  juice.  The  following  table  indicates  the 
results  obtained : — 


Quantity  of  Enzyme. 
Diet. 


Proteolytic. 


BOO 


Bread,  250  grin.       .         .  !       1978  1601 

Milk,  600  cc 1085  432 

Flesh,  100  grm.       .         .  |       1502  648  8000 


Amylolytic.     Fat  Splitting. 


Bread  contains  a  proteid  difficult  of  digestion,  plenty  of  starch, 
and  little  fat.  Milk  contains  an  easily  digested  proteid,  and 
plenty  of  fat,  but  no  starch;  while  flesh  contains  a  com- 
paratively easily  digested  proteid,  no  starch,  and  little  fat. 
The  first  food  causes  a  copious  production  of  trypsin  and 
amylopsin,  and  little  pialyn.  The  second  causes  the  produc- 
tion of  less  trypsin,  little  amylopsin,  but  most  pialyn.  The 
last  causes  a  moderate  production  of  trypsin,  little  amy- 
lopsin, and  a  comparatively  large  amount  of  pialyn. 

As  to  the  mode  of  production  of  these  enzymes,  it  is 
known  that  trypsin  is  not  formed  as  such  in  the  cells,  but 
that  a  forerunner  of  trypsin — trypsinogen — is  produced,  and 
that  this  changes  into  trypsin  after  it  is  secreted.  The  in- 
testinal secretion  contains  something  which  has  been  termed 
enterokinase,  which  has  the  power  of  bringing  about  this 
change,  and  in  all  probability  the  cells  lining  the  ducts  of 
the  pancreas  also  produce  this  or  a  similar  substance. 

It  is  doubtful  whether  the  pancreatic  secretion  contains 


THE  FOOD  AND  DIGESTION  349 

any  true  rennin,  although  it  produces  a  modified  clotting  of 
milk,  under  certain  conditions. 

The  influence  of  the  pancreas  in  the  general  metabolism 
will  be  considered  later  (p.  390). 

3.  Physiology  of  Pancreatic  Secretion. — The  secretion 
of  pancreatic  juice  is  not  constant,  but  is  induced  when 
the  acid  chyme  passes  into  the  duodenum.  This  occurs 
even  when  all  the  nerves  to  the  intestine  have  been 
cut,  and  it  appears  to  be  due  to  the  formation  in  the 
epithelium  lining  the  intestine,  under  the  influence  of  an 
acid,  of  a  material  which  has  been  called  secretin.  This 
is  absorbed  and,  on  being  carried  to  the  pancreas,  stimulates 
it  to  secrete.  It  has  been  shown  that  the  injection  into  the 
blood  of  an  extract  of  the  lining  membrane  of  the  upper  part 
of  the  small  intestine,  made  with  dilute  hydrochloric  acid, 
leads  to  a  flow  of  pancreatic  juice.  This  secretin  is  not 
destroyed  by  boiling,  and  is  soluble  in  strong  alcohol.  It  is 
therefore  not  of  the  nature  of  an  enzyme. 

But  while  secretin  seems  to  play  so  important  a  role, 
it  has  been  found  that  stimulation  of  the  vagus  nerve, 
after  a  latent  period  of  two  minutes,  increases  pancreatic 
secretion,  so  that  it  must  be  concluded  that  the  secretion 
of  the  fluid  is,  to  a  certain  extent,  under  the  control  of  the 
nervous  system. 

C.  Secretion  of  the  Intestinal  Wall  (Succus  Entericus). 

This  is  formed  in  the  Lieberkiihn's  follicles  of  the  intes- 
tine, and  it  may  be  procured  by  cutting  the  intestine  across 
at  two  points,  bringing  each  end  of  the  intermediate  piece  to 
the  surface,  and  connecting  the  ends  from  which  this  piece 
has  been  taken  away.  On  mechanically  irritating  the 
mucous  membrane,  a  pale  yellow  clear  fluid  is  secreted, 
which  contains  native  proteids  and  mucin,  and  is  alkaline 
in  reaction  from  the  presence  of  carbonate  of  soda. 

Action. — The  succus  entericus  contains:  (1)  An  enzyme 
which  splits  some  disaccharids,  as  maltose  and  cane  sugar, 
into  monosaccharids,  but  does  not  seem  to  act  on  lactose. 
(2)  In  the  intestine  of  animals  taking  milk  a  special  zymin, 


350  HUMAN  PHYSIOLOGY 

lactase,  which  splits  inilk  sugar.  (3)  Erepsin,  an  enzyme 
which  seems  to  act  more  powerfully  than  trypsin  in  splitting 
peptone  and  many  other  proteids  into  their  component 
non-proteid  crystalline  constituents  such  as  the  di-amido 
acids  and  non-amido  acids,  e.g.  leucin  and  tyrosin.  The 
object  of  this  is  not  easy  to  understand,  but  it  may  be  that 
the  nitrogen  of  the  proteid  is  largely  treated  as  a  waste 
product.  Vernon  has  shown  that  a  similar  enzyme  is  widely 
distributed  in  the  tissues,  being  specially  abundant  in  the 
kidney.  (4)  Enterokinase — a  zymin  which,  acting  on  tryp- 
sinogen,  converts  it  into  active  trypsin  (p.  348). 

Nervous  Mechanism  of  Secretion. — So  far  very  little  is 
known  on  this  point.  It  has  been  found  that,  when  the  in- 
testine is  ligatured  in  three  places  so  as  to  form  two  closed 
sacs,  if  the  nerves  to  one  of  these  be  divided,  it  becomes  filled 
with  a  clear  fluid  closely  resembling  lymph.  The  dilatation 
of  the  blood-vessels  may  account  for  this  without  secretion 
being  implicated. 

Bacterial  Action  in  the  Alimentary  Canal. 

With  the  food,  water,  and  saliva,  numerous  micro- 
organisms of  very  diverse  character  are  swallowed.  It  has 
been  suggested  that  the  leucocytes  formed  in  the  lymphoid 
tissue  at  the  back  of  the  mouth  and  pharynx,  attack  and 
destroy  such  organisms,  but  so  far  definite  proof  of  this  is 
not  forthcoming.  When  the  food  is  swallowed,  the  micro- 
organisms multiply  for  some  time  in  the  warm  moist  stomach, 
and  certain  of  these,  by  splitting  sugars,  form  lactic  and 
sometimes  acetic  acid.  But  when  sufficient  gastric  juice  is 
poured  out  for  the  hydrochloric  acid  to  exist  free,  the  growth 
of  micro-organisms  is  inhibited,  and  some,  at  least,  are 
killed.  Others  pass  on  into  the  intestine,  and,  as  the  acid  in 
the  chyme  becomes  neutralised,  the  acid-forming  organisms 
begin  to  grow,  and,  by  splitting  the  sugars,  form  lactic  or  acetic 
acid,  and  render  the  contents  of  the  small  intestine  slightly 
acid.  Towards  the  end  of  the  small  intestine,  and  more 
especially  in  the  large  intestine,  the  alkaline  secretions  have 
neutralised  these  acids,  and  in  the  alkaline  material  so  pro- 
duced the  putrefactive  organisms  begin  to  flourish  and  to 


THE   FOOD   AND   DIGESTION 

attack  any  proteid  which  is  not  absorbed — splitting  it  up  and 
forming  among  other  substances  a  series  of  aromatic  bodies, 
of  which  the  chief  are  indol,  skatol,  and  phenol. 
Indol  is  a  derivative  of  ethyl-amido-benzene. 


Amido-benzene. 


H    H    H 

\      I       I       I 
TT   x — N— C— C— H     Ethyl-amido-benzene. 

A  A 


In  Skatol  a  hydrogen  of  the  ethyl  of  indol  is  replaced 
by  CH3. 
Phenol  is 


—  O— H 


By  taking  embryo  guinea-pigs  at  full  time  from  the  uterus 
and  keeping  them  with  aseptic  precautions,  it  has  been  shown 
that  the  absence  of  micro-organisms  from  the  intestine  does 
not  interfere  with  digestion. 

The  most  important  and  abundant  organism  present  in 
the  intestinal  tract  is  the  bacillus  coli  communis,  which  has 
a  certain  power  of  splitting  proteids  and  a  marked  action 
in  producing  acids  from  sugars.  Its  presence  in  water  is 
generally  considered  indicative  of  sewage  contamination. 


352  HUMAN  PHYSIOLOGY 

Fate  of  the  Digestive  Secretions. 

1.  Water. — Although  it  is  impossible  to  state  accurately 
the  average  amount  of  the  various  digestive  secretions  poured 
into  the  alimentary  canal  each  day,  it  must  be  very  consider- 
able, probably  not  far  short  of  3000  ccms.,  or   something 
considerably  more  than  one-half  of  the  whole  volume  of  the 
blood.     Only  a  small  amount  of  this  is  given  off  in  the  faeces, 
and  hence  the  greater  part  must  be  re-absorbed.     There  is 
thus   a   constant   circulation   between   the   blood    and    the 
alimentary  canal,  or  what  may  be  called  an  entero-hcemal 
circulation.     One  portion  of  this  is  particularly  important. 
The  blood  vessels  of  the  intestine  pass  to  the  liver,  and  many 
substances,  when  absorbed  into  the  blood  stream,  are  again 
excreted  in  the  bile  and  thus  are  prevented  from  reaching 
the  general  circulation.      Among  these  substances  are  the 
salts  of  the  bile  acids  and  their  derivatives,  many  alkaloids 
such  as  curarine,  and  in  all  probability  a  set  of  animal  alka- 
loids called  ptomaines  formed  by  putrefactive  decomposition 
of  proteids  in  the  gut.     If,  from  disturbances  in  the  functions 
of  the  liver,  these  are  allowed  to  pass  through  that  organ,  the 
feelings  of  lassitude  and  discomfort  which  are  associated  with 
intestinal  dyspepsia  are  produced.     The  liver  thus  forms  a 
protective  barrier  to  the  ingress  of  certain  poisons. 

2.  Enzymes. — Ptyalin   appears   to   be   destroyed    in   the 
stomach   by   the   hydrochloric   acid.      Pepsin  is  probably 
partly  destroyed  in  the  intestine,  but  it  seems  also  to  be 
absorbed  and  excreted  in  the  urine ;  for,  on  the  addition  of 
hydrochloric  acid,  the  urine  has  a  peptic  action  on  proteids. 
Trypsin  appears  to  be  destroyed  in  the  alimentary  canal ; 
but  the  fate  of  the  other  pancreatic  enzymes  and  of  the 
enzymes  of  the  succus  entericus  is  unknown. 

3.  Bile   Constituents. — 1.    The   bile    salts  are   partly   re- 
absorbed    from    special    parts    of    the     small    intestine — 
glycocholate  of  soda   being   taken   up  in  the  jejunum  and 
taurocholate  in  the  ileum.     The  acids  of  these  salts  are  also 
partly  broken  up.     The  glycocholic  acid  yields  amido-acetic 
acid,  which  is  absorbed  and  passes  to  the  liver  to  be  excreted 
as  urea;  while  the  taurocholic  acid  yields  amido-isethionic 
acid  which  goes  to  the  liver  and  yields  urea  and  probably 


THE   FOOD   AND  DIGESTION  353 

sulphuric  acid.  The  fate  of  the  cholalic  acid  is  not  known, 
but  it  is  supposed  to  be  excreted  in  the  faeces.  2.  The 
pigments  undergo  a  change  and  lose  their  power  of  giving 
Gmelin's  reaction.  They  appear  in  the  faeces  as  what  may 
be  called  stercobilin.  It  is  probably  formed  by  reduction  of 
bilirubin  in  the  intestines  as  the  result  of  the  action  of 
micro-organisms.  3.  The  cholesterin  is  passed  out  in  the 
faeces. 

Faeces. 

The  materials  not  absorbed  from  the  intestine,  whether 
these  are  derived  from  the  food  or  from  the  alimentary 
canal,  are  thrown  off  from  the  rectum  as  the  faeces.  In 
fasting  animals  these  are  passed  at  long  intervals,  and 
consist  of  mucin,  shed  epithelium,  the  various  products 
of  the  bile  constituents,  and  inorganic  salts.  In  feed- 
ing animals  the  amount  and  character  of  the  faeces  de- 
pends largely  upon  the  amount  and  character  of  the  food, 
and  upon  the  bacteria  which  are  growing  in  the  large  in- 
testine. The  unabsorbed  material,  as  it  passes  down  the 
large  intestine,  becomes  inspissated  from  the  absorption  of 
water,  but,  if  much  undigested  matter  is  present,  water 
may  also  be  added,  and  the  consistence  of  the  faeces  may 
thus  be  varied.  In  the  average  condition  they  contain 
about  70  or  80  per  cent,  of  water.  The  colour  is  normally 
brown,  from  the  haematin  of  the  flesh  eaten,  while  the 
sulphide  of  iron  formed  by  the  splitting  of  the  haematin 
compounds  in  the  intestine  may  make  them  darker  in 
colour.  On  a  milk  diet  they  are  light  yellow  in  colour, 
and  if  a  large  excess  of  fatty  food  is  taken,  or  if  fat  is  not 
absorbed,  as  in  jaundice,  they  become  clay  coloured. 

The  derivatives  of  the  bile  pigments  play  but  a  small  part 
in  colouring  the  fasces.  In  infants,  before  bacteria  are 
introduced  and  begin  to  exert  their  reducing  action,  the 
faeces  may  be  green  from  the  presence  of  unaltered  biliverdin. 
The  reaction  of  the  faeces  varies.  Usually  the  outside  of  the 
mass  is  alkaline  from  the  alkaline  secretion  of  the  intestine, 
while  the  inside  is  acid  from  the  free  fatty  acids  and  other 
acids  formed  by  the  action  of  such  acid-forming  bacteria  as 

23 


354  HUMAN   PHYSIOLOGY 

the  bacillus  coli  cominunis.  The  amount  of  solid  faeces 
depends  on  the  amount  of  food — a  fairly  average  amount 
per  diem  is  150  grms.  of  dried  solids.  On  a  vegetable  diet, 
from  the  presence  of  undigested  cellulose,  the  amount  is 
very  much  greater.  The  solids  of  the  faeces  of  a  feeding 
animal  consist  of  the  same  constituents  as  the  faeces  in  a 
fasting  animal,  with  the  addition  of  all  the  undigested  con- 
stituents of  the  food — elastic  and  white  fibrous  tissue,  remains 
of  muscle  fibres,  often  fat  and  the  earthy  soaps  of  the  fatty 
acids ;  and,  when  a  vegetable  diet  is  taken,  the  cellulose  of 
the  vegetable  cells,  and  frequently  starch.  The  cellulose,  by 
stimulating  the  intestine,  is  a  valuable  natural  purgative. 

The  odour  is  due  to  the  presence  of  aromatic  bodies  such 
as  indol  and  skatol. 

Meconium  is  the  name  given  to  the  first  fyeces  passed  by 
the  child  after  birth.  They  are  greenish-black  in  colour, 
and  consist  of  inspissated  bile  and  shed  epithelium  from 
the  intestine. 

Movements  of  the  Intestine. 

These  are  of  two  kinds — myogenic  and  peristaltic.  The 
myogenic  movements  are  slight  rhythmic  contractions  which 
pass  rapidly  along  the  intestine,  and  are  insufficient  to 
drive  on  the  contents,  but  are  probably  of  use  in  churning 
and  mixing  them.  By  feeding  with  food  mixed  with  bismuth, 
and  employing  X  rays,  Cannon  finds  that  the  contents  of  the 
small  intestine  get  broken  up  into  small  segments.  This  is 
possibly  due  to  these  myogenic  movements  (Fig.  149,  p.  339). 
They  occur  when  all  the  nerves  have  been  divided,  and  when 
the  ganglia  in  the  intestinal  walls  have  been  poisoned  with 
nicotine,  and  they  are  therefore  due  to  the  muscle  fibres 
alone. 

The  peristaltic  movements  are  much  more  complex  and 
powerful.  They  consist  of  a  constriction  of  the  muscles, 
which  seems  to  be  excited  by  the  passage  of  the  food,  and 
may  be  caused  by  inserting  a  bolus  of  cotton-wool  covered 
with  vaseline.  Starting  at  the  upper  end  of  the  intestine, 
they  pass  slowly  downwards.  In  front  of  this  contraction 
the  muscular  fibres  are  relaxed,  and  thus  the  contracting 


THE   FOOD   AND   DIGESTION  355 

part  drives  its  contents  into  the  relaxed  part  below.  These 
peristaltic  movements  go  on  after  the  nerves  to  the  gut  are 
cut,  but  they  are  stopped  when  the  ganglia  in  the  wall  of  the 
intestine  are  poisoned  with  nicotine.  It  has  therefore  been 
concluded  that  the  nerve  ganglia  in  the  intestinal  wall  form 
a  local  reflex  mechanism,  which  is  stimulated  by  the  presence 
of  foreign  matter  in  the  intestine,  and  which  brings  about 
the  co-ordinated  contraction  and  relaxation,  which  together 
constitute  a  true  peristalsis. 

But  while  peristalsis  is  thus  independent  of  the  central 
nervous  system,  it  is  nevertheless  controlled  by  it.  The 
splanchnic  nerves  inhibit,  while  the  vagus  to  the  small  intes- 
tine and  upper  part  of  the  large  gut,  and  the  nervi  erigentes 
to  the  lower  part  of  the  large  gut  are  augmentor  nerves, 
increasing  the  peristalsis. 

As  the  contents  of  the  small  intestine  are  forced  through 
the  ileo-csecal  valve,  the  large  intestine  relaxes  to  receive 
them,  and  then,  a  series  of  contractions  passing  from  below 
upwards — an  anti-peristalsis — sets  in  by  which  the  contents 
are  very  thoroughly  churned.  Afterwards  they  are  forced 
downwards  by  tonic  peristaltic  waves. 

The  intestinal  movements  are  inhibited  by  emotions. 


Defsecation. 

By  the  peristalsis  of  the  intestine,  the  matter  not  absorbed 
from  the  wall  of  the  gut  is  forced  down  and  accumulates  in 
the  part  of  the  rectum  which  passes  horizontally  forward 
to  end  in  the  anal  canal,  into  which  it  is  prevented  from 
escaping  by  the  sharp  fold  which  the  last  part  of  the  bowel 
makes,  and  by  the  contraction  of  the  strong  sphincter  ani. 

Defsecation  depends  primarily  on  the  intestinal  peristalsis, 
without  which  it  cannot  be  performed.  When  faeces  accumu- 
late in  the  rectum,  the  mucous  membrane  is  stimulated,  and 
impulses  are  sent  up  to  inhibit  a  centre  in  the  lumbar  region 
of  the  cord  which  keeps  the  sphincter  ani  contracted,  and 
the  sphincter  is  relaxed,  and  the  escape  of  faeces  made  pos- 
sible. In  some  diseases  of  the  cord  this  centre  is  stimulated 
and  cannot  be  inhibited,  and  thus  defaecation  is  interfered 


356  HUMAN   PHYSIOLOGY 

with,  while  in  other  diseases,  when  this  centre  has  been 
destroyed,  the  sphincter  does  not  contract,  and  faeces  may 
escape  continuously.  Normally  the  act  of  defecation  is 
partly  voluntary  and  partly  involuntary.  The  voluntary  part 
of  the  act  consists  in  closing  the  glottis,  and  making  a  forced 
expiration  so  as  to  press  upon  the  contents  of  the  abdomen, 
while  at  the  same  time  the  perineal  muscles  are  relaxed,  and 
the  rectum  straightened,  and  thus  the  contents  are  allowed 
to  pass  into  the  anal  canal.  The  act  is  completed  by  the 
emptying  of  the  rectum  by  the  contraction  of  the  levatores 
ani  muscles. 


III.  ABSORPTION  OF  FOOD 

1.  State  in  which  Food  leaves  the  Alimentary  Canal.— The 

carbohydrates  generally  leave  the  alimentary  canal  as 
monosaccharids ;  but  some  resist  the  action  of  digestion 
more  than  others.  Lactose  seems  to  be  broken  down  in  the 
intestine  only  when  the  special  lactase  is  present  in  the  succus 
entericus,  but  in  all  cases  it  must  be  broken  down  before  it 
reaches  the  liver.  Cane  sugar  when  taken  in  large  excess 
may  also  be  absorbed  and  it  is  then  excreted  by  the  kidneys. 

The  proteids  are  absorbed  as  peptones,  possibly  as  proteoses, 
and  as  the  diamido  acids  and  other  crystalline  compounds 
formed  by  the  action  of  erepsin  (p.  350).  Native  proteids 
may  be  absorbed  unchanged  from  the  lower  bowel,  since 
it  has  been  found  that  when  egg  white  is  injected  into  an 
isolated  part  of  the  rectum  it  disappears  to  a  very  consider- 
able extent. 

The  fats  are  chiefly  absorbed  as  soaps  and  as  fatty  acids, 
and  it  is  very  doubtful  if  they  leave  the  gut  as  fats. 


2.  Mode  of  Absorption  of  Food. — Absorption  does  not 
occur  uniformly  throughout  the  alimentary  canal.  Thus, 
while  sugar  and  peptones  are  absorbed  from  the  stomach, 
water  is  absorbed  only  to  a  small  extent. 

That  absorption  is  not  due  merely  to  a  process  of  ordinary 
diffusion  or  osmosis  is  clearly  indicated  by  many  facts. 

1.  Heidenhain  has  shown  that  absorption  of  water  from 
the  intestine  takes  place  much  more  rapidly  than  diffusion 
through  a  dead  membrane. 

2.  The  relative  rate  of  absorption  of  different  substances 
does  not  follow  the  laws  of  diffusion.    Griibler's  peptone  passes 
more  easily  through  the  intestine  than  glucose,  but  glucose 
passes  more  readily  through  parchment  paper,  while  sodium 

357 


358  HUMAN  PHYSIOLOGY 

sulphate,  which  is  more  diffusible  than  glucose,  is  absorbed 
much  less  readily.  Again,  as  shown  by  Reid,  an  animal  can 
absorb  its  own  serum  under  conditions  in  which  filtration 
into  blood  capillaries  or  lacteals  is  excluded.  In  such  a 
case  osmosis  cannot  play  a  part.  Absorption  is  stopped 
or  diminished  when  the  epithelium  is  removed,  injured, 
or  poisoned  with  fluoride  of  sodium,  in  spite  of  the  fact 
that  this  must  increase  the  facilities  for  osmosis  and 
filtration. 

3.  Channels  of  Absorption. — There  are  two  channels  of 
absorption  from  the  alimentary  canal  (see  Fig.  105,  p.  209)— 
the  veins  which  run  together  to  form  the  portal  vein  of  the 
liver,  and  the  lymphatics  which  run  in  the  mesentery  and, 
after  passing  through  some  lymph  glands,  enter  the  recep- 
taculum  chyli  in  front  of  the  vertebral  column.  From  this, 
the  great  lymph  vessel,  the  thoracic  duct,  leads  up  to  the 
junction  of  the  subclavian  and  innominate  veins,  and  pours 
its  contents  into  the  blood-stream.  The  lymph  formed  in 
the  liver  also  passes  into  the  thoracic  duct. 

1.  Proteids. — Peptones  and  the  further  products  of  their 
digestion  under  the  influence  of  erepsin  are  formed  from 
proteids  in  digestion,  but  the  peptones  undergo  a  change  in 
the  intestinal  wall  before  passing  to  the  tissues,  since  they 
are  not  found  in  the  blood.  That  in  some  altered  condition 
they  leave  the  intestine  by  the  blood  and  not  by  the  lymph 
is  shown  by  the  fact  that  their  absorption  is  not  interfered 
with  by  ligature  of  the  thoracic  duct. 

During  the  digestion  of  proteids  the  number  of  leucocytes  is 
enormously  increased,  sometimes  to  more  than  double  their 
previous  number,  and  in  all  probability  it  is  they  which  carry 
the  products  of  digestion  from  the  intestine.  According  to 
the  observations  of  Pohl,  the  leucocytes  are  derived  from  the 
lymph  tissue  in  the  intestinal  wall,  but  more  recent  experi- 
ments tend  to  show  that  they  come  from  the  bone  marrow, 
being  probably  attracted  to  the  intestine  by  a  positive 
chemiotaxis.  By  breaking  down  in  the  blood-stream  they 
probably  set  free  the  proteids  for  use  in  the  tissues. 

When  an  excess  of  proteids  is  taken  in  the  food,  it  is 
broken  down  in  the  lining  membrane  of  the  gut,  its  nitro- 


THE   FOOD  AND  DIGESTION  359 

genous  part  forming  ammonia  compounds,  probably  carba- 
mate  of  ammonia — 


o 

H  ,! 

\N C— 0- 


H 

H 
H 


Its  non-nitrogenous  part  -probably  yields  carbohydrates, 
since  a  proteid  diet  may  lead  to  the  accumulation  of  these 
substances  in  the  liver.  The  ammonia  compounds  are  car- 
ried to  the  liver  and  there  changed  to  urea,  and  excreted 
as  such.  Thus  the  entrance  of  an  excess  of  nitrogen  into  the 
tissues  is  prevented. 

It  has  been  pointed  out  that  gastric  juice  does  not  dissolve 
the  nucleo-proteids,  but  that  they  are  dissolved  by  the 
pancreatic  juice.  Phosphorus  is  undoubtedly  absorbed  in 
organic  combination,  but  the  mode  of  absorption  and  the 
channels  by  which  it  passes  from  the  intestine  have  not  been 
investigated. 

2.  Carbohydrates.  —  Although   the  chief   monosaccharid 
formed  in  digestion  is  dextrose,  others  are  also  produced — 
laevulose  from  cane  sugar,  and  galactose  from  milk  sugar. 
All  these  are  absorbed  in  solution,  and  are  carried  away  in 
the  blood  by  the  portal  vein. 

3.  Fats. — It  was  for  long  thought  that  the  fats  are  absorbed 
as  a  fine  emulsion  ;  but  the  most  recent  investigations  seem 
to  indicate  that  after  being  split  up  into  the  component  acids 
and  glycerin,  they  pass,  as  soluble  soaps  or  as  fatty  acids 
soluble  in  the  bile,  through  the  borders  of  the  intestinal 
epithelium.     Here  they  appear  to  be  again  converted  into 
fats  by  a  synthesis  of  the  acid  with  glycerin.     Fine  fatty 
particles   are  found   to  make   their  appearance  in  the  cells 
at  some  distance  from  the  free  margin  and  to  increase  in 
size.     They  are  passed  on  from  the  cells,  through  the  lymph 
tissue  of  the  villi,  into  the  central  lymph  vessels,  and  thus 
on  through  the  thoracic  duct  to  the  blood-stream.     Unlike 
the  proteids  and  carbohydrates,  they  are  not  carried  directly 
to  the  liver. 


IV.  FATE  OF  THE  FOOD  ABSORBED 

THE  food  absorbed  may  be  (A)  used  immediately  as  a  source 
of  energy,  for  (1)  the  Construction  or  Reconstruction  of 
Tissues ;  (2)  the  Production  of  Mechanical  Work ;  (3)  the 
Production  of  Heat ; 

Or  (B)  it  may  be  stored  for  future  use  in  the  body. 

The  processes  of  construction  and  repair  of  the  tissues 
and  the  production  of  mechanical  work  have  already  been 
considered  (p.  67  et  seq.\  and  the  production  of  heat  and  the 
regulation  of  temperature  may  now  be  dealt  with. 

I.  PRODUCTION  OF  HEAT  AND  REGULATION  OF 
TEMPERATURE. 

1.   Production  of  Heat. 

A.  Muscle. — The  production  of  heat  in  muscle  has  been 
already  studied  (p.  G3).     It  has   been  shown  that  muscle, 
from  its  great  bulk  and  constant  activity,  is  the  main  source 
of  heat  in  the  body.      Not  only  may  it   be   demonstrated 
that   the   temperature   of  contracting  muscle   rises,  but  it 
has  been  found  that  the  temperature  of  blood  coming  from 
the   muscles  is   slightly  higher   than   that  of  blood  going 
to  them.     Muscular  exercise  raises  the  temperature  of  the 
body,  and  the  shivering  fit  which  is  induced  by  exposure  to 
cold  is  really  a  reflex  reaction  by  which  heat  production  is 
increased.     Drugs  which  interfere  with  muscular  contraction, 
such  as  curare,  diminish  the  temperature,  and  young  animals, 
before  their  muscular  tissues   become   active,  have   a   low 
temperature  unless  kept  in  a  warm  atmosphere. 

B.  Glands. — Wherever  chemical  change  goes  on  in  proto- 
plasm, heat  is  liberated.     Therefore  in  glands  during  activity 
a  certain  amount  of  heat  is  produced,  but  the  production  in 
them  is  trivial  when  compared  with  the  production  in  muscle. 


THE  FOOD   AND  DIGESTION  361 

During  the  period  of  active  digestion  the  temperature  of  the 
blood  coming  from  the  liver  may  be  nearly  1°  C.  higher  than 
that  of  the  blood  going  to  the  organ.  The  liver  alone  among 
glandular  structures  contributes  an  appreciable  amount  of 
heat  to  the  body,  since  the  amount  of  blood  passing  through 
the  organ  is  large,  and  thus  a  considerable  amount  of  heat  is 
derived  from  it. 

C.  Brain. — Some  physiologists  have  maintained  that  the 
fact  that  the  temperature  of  the  brain  rises  during  cerebral 
activity  indicates  that  the  chemical  changes  going  on  are 
sufficient  to  yield  a  certain  amount  of  heat.  But  it  is  more 
probable  that  the  rise  of  temperature  is  due  to  the  increased 
flow  of  blood  through  the  organ,  since  a  study  of  the  blood 
gases  in  the  brain  gives  no  indication  of  any  marked  increase 
of  chemical  change  during  periods  of  increased  cerebral 
action. 

2.  Regulation  of  Temperature. 

Since  heat  is  constantly  being  produced,  the  temperature 
of  the  body  would  tend  to  rise  higher  and  higher,  were 
there  not  some  arrangement  by  which  just  as  much  heat  is 
eliminated  as  is  produced,  and  by  which  the  temperature  is 
thus  kept  constant. 

Elimination  of  Heat. — Heat  is  got  rid  of  by  three 
channels.  A.  Skin. — Since  the  body  is  generally  warmer 
than  the  surrounding  air,  heat  is  constantly  lost  by  conduc- 
tion, convection,  and  radiation,  and  the  extent  of  this  loss 
depends  mainly  upon  the  difference  between  the  temperature 
of  the  body  and  that  of  the  air.  Radiation  plays  the  most 
important  part  when  a  person  is  sitting  quiet  in  still  air ; 
conduction  and  convection  when  the  exchange  of  air  over 
the  surface  is  rapid.  The  temperature  of  the  skin  is  increased 
when,  from  dilatation  of  the  cutaneous  vessels,  more  blood  is 
brought  to  the  surface,  and  conversely  it  is  lowered  by  con- 
striction of  these  vessels.  The  influence  of  variations  in  the 
temperature  of  the  air  is  generally  minimised  in  man  by  the 
covering  of  clothes,  and  in  animals  by  the  covering  of  fur  or 
feathers,  which  retains  a  stationary  layer  of  air  at  about  25° 
to  30°  C.  over  the  skin.  It  has  been  calculated  that  over 


362  HUMAN  PHYSIOLOGY 

70  per  cent,  of  all  the  heat  is  lost  by  conduction  and 
radiation. 

By  the  evaporation  of  sweat,  heat  is  rendered  latent,  and 
is  taken  from  the  body,  which  is  thus  cooled  just  as  the  hand 
may  be  cooled  by  allowing  ether  to  evaporate  upon  it.  If 
the  amount  of  sweat  vaporised  is  known,  it  is  possible  to 
calculate  the  amount  of  heat  removed  from  the  body  hi 
this  way.  The  loss  is  comparatively  small — only  about 
14  per  cent,  of  the  whole.  The  extent  depends  upon  the 
rapidity  with  which  evaporation  goes  on,  and  this  is  governed 
by  the  amount  of  sweat  secreted,  and  by  the  dryness  and 
temperature  of  the  atmosphere.  Thus  a  warm  dry  climate 
is  better  borne  than  one  which  is  warm  and  moist,  since  in 
the  former  the  loss  of  heat  by  evaporation  is  so  much  freer. 
Of  the  various  factors  increasing  sweat  secretion,  heat  is 
probably  the  most  important. 

Since  the  temperature  of  the  skin  is  governed  by  the  state 
of  the  cutaneous  vessels,  and  the  amount  of  sweat  produced 
by  the  state  of  the  sweat  glands,  and  since  both  of  these 
are  under  the  control  of  the  nervous  system,  the  elimina- 
tion of  heat  from  the  skin  is  presided  over  by  a  nervous 
mechanism. 

B.  Respiratory  Passages. — By  conduction  and  radiation, 
and  by  evaporation  from  the  respiratory  passages,  about  10 
per  cent,  of  the  heat  is  got  rid  of  in  man.     In  the  dog  and 
some  other  animals,  the  proportion  of  heat  eliminated  in 
this  way  is  considerably  greater. 

C.  Urine  and   Faeces. — Since  these  are  warmer  than  the 
surrounding  air,  a  certain  amount  of  heat  is  lost  through 
them.     The   amount  is   small— something  less   than  2  per 
cent,  of  the  whole. 

Temperature. — In  all  higher  animals,  the  loss  of  heat  and 
the  production  of  heat  are  so  nicely  balanced  that  the  tem- 
perature of  the  body  remains  fairly  constant  under  all  con- 
ditions. If  an  extra  amount  of  heat  is  produced,  say  in 
muscular  exercise,  it  is  at  once  eliminated  by  the  skin,  and, 
if  the  body  is  exposed  to  a  low  temperature,  loss  of  heat  is 
rapidly  checked  by  contraction  of  the  cutaneous  vessels  and 
diminished  activity  of  the  sweat  glands. 


THE  FOOD  AND  DIGESTION  363 

Since  heat  is  constantly  being  given  off,  the  temperature  at 
the  surface  of  the  body  is  always  lower  than  the  temperature 
in  the  interior.  The  temperature  of  the  rectum  may  be  taken 
as  a  measure  of  the  internal  temperature. 

The  mean  daily  temperature  of  a  healthy  man  is : — 

0  c.  °  F. 

Kectum         .         .         .     37*2  98-96 

Axilla    ....     36-9  9845 

Mouth  ....     36-87  98-36 

But  the  temperature  varies  throughout  the  course  of  life. 

°c. 

Infant  .  .  .  .  37 '5 
Under  25  years  .  .  37*2 
About  40  .  .  .  37-1 
Old  Age  .  .  .  37-2  to  37*5 

It  also  varies  throughout  the  course  of  the  twenty-four 
hours,  and  since  this  is  a  matter  of  great  importance  in 


FIG.  150. — Chart  of  the  daily  variation  of  temperature  in  the  normal  human  subject 
in  degrees  Centigrade.     (RiCHET. ) 

medicine,   it    has    been    very   carefully   studied    by   many 
observers.     The  difference   is   not  more  than  1°  C.     It  is 
lowest  in  the  early  morning  and  highest  in  the  afternoon. 
Under   all   normal   conditions   the   temperature  of  man 


364  ,  HUMAN   PHYSIOLOGY 

undergoes  only  small  variations,  because  the  balance  between 
production  of  heat  and  elimination  of  heat  is  so  well  main- 
tained. But  under  abnormal  conditions  the  balance  is 
frequently  upset.  Thus  severe  muscular  work  causes  a 
temporary  rise  of  temperature,  because  heat  elimination  does 
not  quite  keep  pace  with  heat  production.  Exposure  to  very 
high  temperatures  may  cause  a  slight  rise  of  temperature, 
while  exposure  to  excessive  cold  may  cause  a  slight  fall; 
but  unless  in  the  case  of  those  unable  to  use  their  muscles — 
e.g.  in  those  suffering  from  alcoholic  poisoning — the  change 
is  small. 

Although  the  normal  variation  of  temperature  is  so  small, 
life  may  be  sustained  when  the  temperature  falls  for  a  time 
to  about  25°  C.,  or  rises  to  nearly  43°  C.  Cases  are  recorded  in 
which  it  has  even  risen  to  46'6°  C.,  without  death  supervening. 

While  the  higher  "warm-blooded  animals,"  mammals  and 
birds,  maintain  a  constant  temperature,  the  lower  vertebrates, 
"  cold-blooded  animals,"  reptiles,  amphibia  and  fishes,  do  not 
do  so,  and  their  temperature  varies  with  that  of  the  surround- 
ing medium. 

But  even  in  mammals  the  mechanism  for  the  regulation  of 
temperature  is  not  absolutely  perfect,  and  in  every  species 
of  animal  there  is  a  limit  to  the  power  of  adjustment. 

Mammals  which  hibernate  become  for  the  time  "  cold- 
blooded animals,"  and  lose  their  power  of  regulating  their 
temperature. 

The  regulation  of  temperature  may  be  effected  either 
by  modifying  heat  production,  or  by  altering  the  rate  of 
elimination. 

Heat  production  is  voluntarily  modified  when  muscular 
exercise  is  taken  during  exposure  to  cold,  and  involuntarily 
when  muscles  are  set  in  action  by  a  shivering  fit.  There  is, 
however,  no  evidence  of  the  existence  of  a  special  nervous 
mechanism  presiding  over  heat  production  in  muscle. 

But  it  is  not  so  much  by  changes  in  the  rate  of  heat  pro- 
duction, as  by  alteration  in  heat  elimination  through  the 
skin,  that  the  temperature  is  kept  uniform.  The  nerves  to 
the  cutaneous  vessels,  and  to  the  sweat  glands,  are  the  great 
controllers  of  temperature.  It  is  through  failure  of  this 
mechanism  under  the  action  of  the  toxins  of  micro-organisms 


THE  FOOD   AND   DIGESTION  365 

that  heat  elimination  is  diminished,  and  the  temperature  is 
raised  in  fevers. 

It  is  not  necessary  to  assume  that  there  is  a  special  heat 
regulating  nervous  mechanism,  since  the  nervous  arrange- 
ments presiding  over  the  vessels  and  glands  of  the  skin  are 
capable  of  immediately  responding  to  change  of  condition 
calling  for  their  intervention. 

II.  STOKAGE  OF  SURPLUS  FOOD. 

This  storage  takes  place  chiefly  in  three  situations :  (1) 
Fatty  tissue ;  (2)  muscle ;  (3)  liver. 

1.  In  Fatty  Tissues. — In  most  mammals  the  chief  storage 
of  surplus  food  is  in  the  fatty  tissues. 

(1)  That  the  fat  of  the  food  can  be  stored  in  them  is 
shown  by  the  fact  that  the  administration  of  large  amounts 
of  fats  different  from  those  of  the  body  leads  to  their  appear- 
ance in  those  tissues. 

(2)  Fats  are  also  formed  from  the  carbohydrates  of  the 
food.     Feeding  experiments  upon  pigs  and  other  animals 
have  definitely  proved  that  sugary  foods  are  changed  to  fat 
in  the  body  and  stored  in  that  form.     The  following  may  be 
given  as  an  example  of  such  experiments.     Two  young  pigs 
of  a  litter  were  taken,  and  one  was  killed  and  analysed.     The 
other  was  fed  for  weeks  on  maize,  the  amount  eaten  being 
weighed   and   the  excretion  of  nitrogen  by  the  pig  being 
determined.     The  animal  was  then  killed  and  analysed,  and 
it  was  found  that  the  fat  gained  was  more  than  could  be 
produced  from  the  fat  and  proteid  of  the  food  eaten.     It  must 
therefore  have  been  formed  from  the  carbohydrates. 

(3)  There  is  good  evidence  that  in  excessive  proteid  feed- 
ing the  non-nitrogenous  part  of  the  proteid  molecule  may  be 
stored  as  fat,  at  least  in  carnivorous  animals.     Thus,  a  dog 
fed  on  lean  meat  does  not  lose  all  his  fat.     It  has  recently 
been  maintained  that  the  evidence  in  favour  of  the  formation 
of  fats  directly  from  proteids  is  unsatisfactory ;  but  since 
proteids  yield  carbohydrates,  and  since  carbohydrates  form 
fats,  it  must  be  admitted  that  proteids  may  be  a  source 
of  fats. 

2.  In  Muscle. — Some  animals,  as  the   salmon,  store  fats 


366  HUMAN  PHYSIOLOGY 

within  their  muscle  fibres ;  but  in  mammals  such  a  storage 
is  limited  in  amount.  The  salmon  also  stores  surplus  proteid 
material  in  the  muscles,  and  mammals  too  appear  to  do  the 
same.  How  far  a  passive  storage  may  occur  is  not  known, 
but  feeding  experiments  on  mammals  indicate  that  only  a 
small  amount  of  proteid  can  be  accumulated.  On  the  other 
hand,  the  experience  of  athletic  training  shows  that  the 
muscles  may  be  enormously  increased  by  the  building  up  of 
the  proteid  of  the  food  into  their  protoplasm.  Glycogen  also 
is  stored  in  the  muscles. 

3.  In  the  Liver. — The  liver  is  a  storehouse  of  carbohydrates 
and  fats  (p.  368).  Lecithin  is  always  present  in  the  liver, 
even  in  prolonged  fasting. 

III.  THE   LIVER   IN   RELATIONSHIP  TO   ABSORBED 
FOOD   AND   TO  THE  GENERAL   METABOLISM. 

The  liver  develops  as  a  couple  of  diverticula  from  the 
embryonic  gut,  and  is  thus  primarily  a  digestive  gland.  But 
in  mammals,  early  in  fcetal  life,  it  comes  to  have  important 
relationships  with  the  blood  going  to  nourish  the  body  from 
the  placenta.  In  invertebrates  it  remains  as  a  part  of  the 
intestine  both  structurally  and  functionally.  The  vein  bring- 
ing the  blood  from  the  mother  breaks  up  into  a  series  of 
capillaries  in  the  young  liver,  and  in  these  capillaries,  for  a 
considerable  time,  the  development  of  the  cells  of  the  blood 
goes  on.  Soon  the  liver  begins  to  secrete  bile,  while  animal 
starch  and  fat  begin  to  accumulate  in  its  cells.  Gradually 
the  formation  of  blood  cells  stops,  and  the  mass  of  liver  cells 
become  larger  in  proportion  to  the  capillaries.  As  the  foetal 
intestine  develops,  the  vein  bringing  blood  from  it — the 
portal  vein — opens  into  the  capillary  network  of  the  liver,  so 
that,  when  at  birth  the  supply  of  nourishment  from  the 
placenta  is  stopped,  the  liver  is  still  associated  with  the  blood 
bringing  nutrient  material  to  the  tissues. 

1.  Relation  to  Carbohydrates — Glycogenic  Function.— 
Claude  Bernard  discovered  that  there  is  a  constant  forma- 
tion of  sugar  in  the  liver.  On  account  of  this  constant 
supply,  even  when  an  animal  undergoes  a  prolonged  fast, 
the  amount  of  sugar  in  the  blood  does  not  diminish.  In 


THE  FOOD  AND   DIGESTION  367 

starvation  there  are  only  two  possible  sources  of  this  glucose 
—the  fats  and  the  proteids  of  the  tissues.  There  is  no  con- 
clusive evidence  that  fats  can  be  changed  to  sugar  in  the 
liver,  although  it  is  difficult  to  explain  the  large  amount  of 
sugar  which  is  excreted  in  phloridzin  poisoning,  unless  it  is 
formed  from  fats.  That  proteids  are  a  source  of  sugary 
substances  is  shown  by  the  accumulation  of  glycogen  in  the 
liver  in  animals  fed  upon  proteids,  and  by  continued  excre- 
tion of  glucose  in  the  urine  of  a  dog  without  its  pancreas, 
and  fed  exclusively  on  proteids.  It  is  therefore  probable 
that  in  starvation  the  proteids  of  the  body  are  broken  down 
and  their  non-nitrogenous  part  changed  to  sugar.  But,  not 
only  does  the  liver  manufacture  sugar  for  the  tissues  in  star- 
vation, but,  when  the  supply  of  sugar  is  in  excess  of  the 
demands  of  the  tissues,  it  stores  it  as  a  form  of  starch — 
glycogen  (see  p.  318) — and  gives  it  out  as  sugar  as  that 
substance  is  required.  On  a  carbohydrate  diet  the  accumu- 
lation of  glycogen  in  the  liver  is  very  great ;  but  even  on  a 
proteid  diet,  in  dogs  at  least,  a  smaller  accumulation  takes 
place.  The  observation  that  various  monosaccharids  are 
stored  as  the  same  form  of  glycogen  shows  that  they  must 
first  be  assimilated  by  the  liver  protoplasm  and  then  con- 
verted to  glycogen,  the  process  being  one  of  synthesis. 

The  way  in  which  glycogen  is  again  changed  to  sugar  is 
more  doubtful.  The  fact  that  the  liver  after  treatment  with 
alcohol  can  change  glycogen  to  glucose,  has  induced  some 
physiologists  to  believe  that  it  is  by  an  enzyme  that  this 
conversion  goes  on  during  life.  But  it  has  been  shown  (1) 
that  the  injection  of  methylene  blue,  which  poisons  proto- 
plasm but  does  not  interfere  with  the  action  of  enzymes 
checks  the  conversion,  and  (2)  that  stimulating  the  splanchnic 
nerves  going  to  the  liver  increases  the  conversion  without 
increasing  the  amylolytic  enzyme  in  the  liver  or  blood. 
It  is  therefore  probable  that  the  conversion  results  from 
chemical  changes  in  the  protoplasm  which  are  controlled  by 
the  nerves  of  the  liver. 

If  more  sugar  is  taken  than  the  liver  can  deal  with,  it  then 
passes  on  into  the  general  circulation,  and  is  excreted  in  the 
urine.  Every  individual  has  a  certain  power  of  oxidising 
and  using  sugar,  and  most  persons  can  dispose  of  about  200 


368  HUMAN   PHYSIOLOGY 

grms.  at  a  time.  But  the  carbohydrate  capacity  varies 
greatly,  and  even  in  the  same  individual  it  is  different  under 
different  conditions.  When  the  glycogen  stored  in  the  liver 
is  changed  to  glucose  more  quickly  than  is  required  by  the 
tissues,  the  glucose  accumulates  in  the  blood  and  is  excreted 
in  the  urine  -(glycosuria).  This  is  seen  in  Bernard's  experi- 
ment of  puncturing  the  floor  of  the  fourth  ventricle  at  its 
posterior  part  in  a  rabbit.  If  glycogen  is  abundant  in  the 
liver,  glycosuria  results. 

Another  way  in  which  sugar  may  be  made  to  appear  in 
the  urine  is  by  injecting  phloridzin.  But  since  under  the 
influence  of  this  drug  the  sugar  in  the  blood  is  decreased,  it 
must  be  concluded  that  it  acts  by  causing  the  kidneys  to 
excrete  glucose  too  rapidly,  so  that  it  is  not  available  for  the 
tissues. 

The  injection  of  large  doses  of  extract  of  the  suprarenal 
bodies  causes  a  glycosuria  with  an  increase  of  sugar  in  the 
blood ;  but  so  far  it  is  not  known  whether  the  condition  is 
one  of  increased  production  of  sugar  or  of  diminished 
utilisation  (p.  385). 

Removal  of  the  pancreas  also  causes  glycosuria  (p.  390). 

2.  Relation  to  Fats. — Although  the  fats  are  not  carried 
directly  to  the  liver,  as  are  proteids  and  carbohydrates,  they 
are  stored  in  large  amounts  in  the  liver  of  some  animals — 
e.g.  the  cod   among  fishes   and  the  cat  among   mammals. 
Animals  which  have  little  power  of  storing  fat  generally 
throughout  the  muscles  and  other  tissues,  seem  to  have  a 
marked  capacity  for  accumulating  it  in  the  liver.     Even  in 
starvation  the  fats  do   not   disappear   from   the  liver,  and 
throughout  all  conditions  of  life  a  fairly  constant  amount  of 
lecithin — a  phosphorus   and    nitrogen    containing   fat   (see 
p.  78) — is  present  in  the  liver  cells.     Lecithin  is  an  inter- 
mediate stage  in  the  formation  of  the  more  complex  nucleins 
of  living  cells,  and  the  formation  of  lecithin  in  the  liver  by 
the  synthesis  of  glycerin,  fatty  acids,  phosphoric  acid,  and 
cholin  is  probably  a  first  step  in  the  construction  of  these 
nucleins.      If  this  is  so,  the  fat  of  the  liver  must  play  an 
important  part  in  retaining  and  fixing  phosphorus  in  the 
body. 

3.  Relation  to  Proteids. — Along  with  the  intestinal  wall, 


THE   FOOD   AND   DIGESTION  369 

the  liver  regulates  the  supply  of  proteids  to  the  body.  A 
study  of  the  chemical  changes  in  muscle  has  shown  that  the 
waste  of  proteid  is  normally  small  in  amount,  and  that  a 
great  part  of  the  nitrogen  is  capable  of  being  used  again  if  a 
supply  of  oxygen  and  carbonaceous  material  is  forthcoming 
(see  p.  72).  Hence  the  demand  for  nitrogen  in  the  muscles 
is  small,  and  for  this  reason,  apparently,  any  excess  of  proteid 
in  the  food  is  decomposed  either  by  erepsin  or  by  the  intes- 
tinal wall  into  ammonia  compounds,  which  are  changed  into 
urea  in  the  liver. 

Urea,  the  chief  waste  substance  excreted  in  the  urine,  is 
the  bi-amide  of  carbonic  acid. 

O  H,         °          H 

VNT    f1    N"/ 
H-O-C-O-H  H/  \H 

It  contains  46-6  per  cent,  of  nitrogen.  It  is  a  white 
substance  crystallising  in  long  prisms*  It  is  very  soluble 
in  water  and  alcohol — insoluble  in  ether.  With  nitric  and 
oxalic  acids  it  forms  insoluble  crystalline  salts.  It  is  readily 
decomposed  into  nitrogen,  carbon  dioxide  and  water  by 
nitrous  acid  and  by  hypobromite  of  soda  in  excess  of  soda. 

Urea  is  chiefly  formed  in  the  liver. — That  it  is  not 
produced  in  the  kidneys  is  shown  by  the  following  facts : 
(1)  When  these  organs  are  excised,  urea  accumulates  in  the 
blood.  (2)  When  carbonate  of  ammonia  is  added  to  blood 
artificially  circulated  through  the  kidney  of  an  animal  just 
killed,  no  urea  is  formed. 

That  it  is  not  formed  in  the  muscles  is  shown — (1)  By  the 
absence  of  a  definite  increase  in  urea  formation  during  mus- 
cular activity ;  (2)  by  the  fact  that  when  blood  containing 
carbonate  of  ammonia  is  streamed  through  muscles,  urea  is 
not  produced. 

That  it  is  formed  in  the  liver  is  indicated — (1)  By  the  fact 
that  when  an  ammonia  salt  such  as  the  carbonate  dissolved 
in  blood  is  streamed  through  the  organ,  it  is  changed  to 
urea ;  (2)  by  the  observation  that,  when  the  liver  is  cut  out 
of  the  circulation,  the  urea  in  the  urine  rapidly  diminishes, 
and  ammonia  and  lactic  acid  take  its  place. 

The  exclusion  of  the  liver  from  the  circulation  in 

24 


370  HUMAN   PHYSIOLOGY 

mammals  is  difficult,  because,  when  the  portal  vein  is  liga- 
tured, the  blood  returning  to  the  heart  tends  to  accumulate 
in  the  great  veins  of  the  abdomen.  But  this  difficulty  has 
been  overcome  by  Eck,  who  devised  a  method  of  uniting  the 
peripheral  end  of  the  divided  portal  vein  with  the  inferior 
vena  cava,  and  of  thus  allowing  the  blood  to  return  from  the 
abdomen  to  the  heart. 

Source  of  Urea. — Urea  is  produced  from  the  proteids  of 
the  food  and  tissues.  The  manner  in  which  excess  of  proteid 
in  the  food  is  broken  down  into  ammonia  compounds  and 
sent  to  the  liver  has  been  already  described  (p.  369).  But 
the  fact  that  even  in  starvation  urea  is  produced,  seems  to 
indicate  that  the  initial  stages  of  decomposition  of  proteids 
may  go  on  elsewhere  than  in  the  -intestinal  wall.  The  fate  of 
haemoglobin  tends  to  show  that  the  whole  process  may  be 
conducted  in  the  liver  cells.  When  haemoglobin  is  set  free 
from  the  corpuscles,  the  nitrogen  of  its  proteid  part  is 
changed  to  urea,  while  the  pigment  part  is  deprived  of  its 
iron  and  excreted  as  bilirubin.  Whether  the  proteids  of 
muscle  and  other  tissues  are  thus  directly  dealt  with,  or 
whether  the  initial  stages  of  decomposition  go  on  outside  the 
liver  is  not  known.  It  is  probable  that  lactate  of  ammonia 
is  produced  in  muscle,  and  that  this  is  converted  into  urea 
in  the  liver. 

Speculations  as  to  the  way  in  which  the  proteid  molecule 
is  broken  down  are  of  little  value.  It  is  one  thing  to  show 
that  urea  may  be  formed  in  a  particular  way  outside  the 
body,  but  quite  another  to  prove  that  it  is  formed  in  that 
particular  way  in  living  protoplasm. 

The  nitrogen  excreted  is  not  all  in  the  form  of  urea,  but 
some  is  combined  in  ammonia  salts,  in  uric  acid  and  other 
purin  bodies  (see  p.  397),  and  in  creatinin.  In  the  mam- 
malian body  ammonia  and  the  purin  bodies  can  be  changed 
into  urea,  and  it  is  probable  that  the  small  amounts  of  these 
substances  which  appear  in  the  urine  have  simply  escaped 
this  conversion.  Certain  drugs  (alcohol,  sulphonal,  &c.)  and 
toxins  (diphtheria)  markedly  decrease  their  conversion  into 
urea  and  so  increase  their  quantity  in  the  urine.  Although 
urea  may  be  prepared  from  creatin,  there  is  no  evidence  that 
such  a  process  goes  on  in  the  body. 


THE  FOOD  AND  DIGESTION  371 

After  the  nitrogenous  portion  of  the  proteid  molecule  is 
split  up,  the  liver  has  the  further  power  of  turning  the  non- 
nitrogenous  part  into  sugar,  and  either  sending  it  to  the 
tissues  or  storing  it  as  glycogen. 

Summary  of  the  Functions  of  Liver. — The  functions  of  the 
liver  may  be  briefly  summarised  as  follows :  (1)  It  regulates 
the  supply  of  glucose  to  the  body  (a)  by  manufacturing  it 
from  proteids  when  the  supply  of  carbohydrates  is  insufficient, 
and  (b)  by  storing  it  as  glycogen  when  the  supply  of  carbo- 
hydrates is  in  excess,  giving  it  off  afterwards  as  required. 
(2)  Along  with  the  intestinal  wall  it  regulates  the  supply  of 
proteids  to  the  body,  by  decomposing  any  excess,  and  giving 
off  the  nitrogen  as  urea,  &c.  (3)  It  regulates,  in  many  animals 
at  least,  the  supply  of  fat  to  the  body  by  storing  any  excess. 
(4)  It  regulates  the  number  of  erythrocytes  by  getting  rid  of 
waste  haemoglobin  and  retaining  the  iron  for  further  use. 
(-5)  From  the  part  it  plays  in  the  entero-hepatic  circulation, 
it  protects  the  body  against  certain  poisons  by  excreting 
them  in  the  bile. 


V.  GENERAL   METABOLISM. 

Having  considered  how  the  food  is  digested  and  absorbed, 
and  how  it  is  then  either  stored  or  at  once  used  (a)  for 
building  up  and  repairing  the  tissues,  or  (6)  as  a  source  of 
energy,  the  rate  at  which  the  various  chemical  changes  go 
on  and  the  factors  modifying  them  may  be  dealt  with. 

The  changes  in  the  two  great  constituents  of  the  body — 
proteids  and  fats — have  to  be  separately  studied. 

1.  Method  of  Investigating. 

A.  Proteid  Metabolism. — The  amount  of  proteid  used  in 
the  body  is  readily  calculated  from  the  amount  of  nitrogen 
excreted,  since,  under  normal  conditions,  unless  nitrogen 
in  some  unusual  combination  is  being  taken,  it  is  derived 
entirely  from  the  proteids  in  the  body.  Proteids  contain 
16  per  cent,  of  nitrogen,  and  hence  each  grm.  of  nitrogen 
excreted  is  derived  from  6'25  grms.  of  proteid. 

The  nitrogen   is   almost   entirely  excreted  in   the  urine. 


372 


HUMAN   PHYSIOLOGY 


Only  a  small  amount  escapes  by  the  bowels  and  skin,  and 
hence  only  when  very  accurate  observations  are  desired  is  it 
necessary  to  analyse  the  faeces  and  sweat. 

Since  nucleo-proteids  form  so  important  a  constituent  of 
living  matter,  it  is  sometimes  desirable  to  study  the  chemical 
changes  which  they  are  undergoing.  To  do  this  the  excre- 
tion of  phosphorus  and  the  purin  bases  must  be  investigated. 
But  it  is  difficult  to  arrive  at  reliable  conclusions,  because 
there  are  other  phosphorus- containing  substances  besides 
nucleins  in  the  body — e.g.  the  bones  ;  and  secondly,  the 
purin  bodies  all  tend  to  be  converted  into  urea  before  being 
excreted. 

B.  Metabolism  of  Fats. — Proteids  contain  nearly  three 
and  a  half  times  as  much  carbon  as  nitrogen,  and  hence, 
when  broken  down,  for  each  grm.  of  nitrogen  excreted, 
3 '4  grms.  of  carbon  are  given  off. 

The  carbon  is  chiefly  excreted  from  the  lungs  as  carbon 
dioxide,  and  in  this  form  it  may  be  collected  and  estimated. 

Any  excess  of  carbon  excreted  over  3-4  times  the  amount 
of  nitrogen  given  oft',  must  be  derived  from  the  fats  of  the 
body  or  from  the  fats  and  carbohydrates  taken  in  the  food. 
Any  carbon  retained  in  the  body,  apart  from  that  in  proteids, 
is  stored  ultimately  as  fat,  and  since  carbon  constitutes 
76*5  per  cent,  of  fats,  the  amount  of  fat  is  calculated  by 
multiplying  the  carbon  by  1/3. 

The  following  tabular  example  of  an  investigation  of  the 
metabolism  may  be  given  :— 


Intake  in  Grammes. 

Output. 

c. 

X. 

C. 

N. 

Proteids      . 

100 

54 

16 

Fats   .... 

100 

76 

Carbohydrates    . 

400 

200 

... 

... 

330 

16 

300 

14 

Two  grms.  of  nitrogen  are  retained  as  proteid ;  that  is,  2  x 
6*25  =  12-5  grms.  of  proteid — are  being  daily  laid  on.  Thirty 
grms.  of  carbon  are  also  retained  in  the  body,  and  of  this 


THE  FOOD  AND  DIGESTION  373 

3'4x2  =  6'8  grms.  are  combined  with  the  nitrogen  in  the 
proteid.  The  remainder,  26'6  grms.,  go  to  form  fats,  the 
amount  of  which  is  26'6  x  1*3  =  34'6  grms.  of  fat. 

2.  Metabolism  during  Fasting. 

When  the  usual  supply  of  energy  in  the  food  is  cut  off, 
the  animal  liberates  the  energy  required  by  oxidising  its  own 
stored  material  and  tissues.  This  is  shown  by  the  fact  that 
the  animal  loses  weight  and  goes  on  excreting  carbon  dioxide, 
urea,  and  the  other  waste  products  of  the  activity  of  the 
tissues. 

Several  prolonged  fasts  have  been  undertaken  by  men,  and, 
in  one  or  two  of  these,  careful  observations  have  been  made 
by  physiologists.  It  has  been  found  that  during  the  first  day 
or  two  of  a  fast,  the  individual  goes  on  using  proteids  and 
fats  at  something  like  the  same  rate  as  he  did  while  taking 
food,  but  that  gradually  he  uses  less  and  less  proteid  each 
day.  This  is  well  shown  in  the  case  of  Succi,  who  underwent 
a  fast  of  thirty  days. 

Day  of  Fast.  Proteid  used  in  Grms.  Fat  used. 

1st  104  Not  estimated. 

10th  51  170 

20th  33  170 

29th  31  163 

It  is  from  the  stored  fats  that  the  energy  is  chiefly  derived, 
and  the  result  of  this  is  that  before  death  the  fats  of  the  body 
are  largely  used  up.  The  proteid-containing  tissues  waste 
more  slowly  and  waste  at  different  rates,  the  less  essential 
being  used  up  more  rapidly  than  the  more  essential,  which, 
in  fact,  live  upon  the  former.  In  cats  deprived  of  food  till 
death  supervened  the  heart  and  central  nervous  system  had 
hardly  lost  weight ;  the  bones,  pancreas,  lungs,  intestines,  and 
skin  each  had  lost  between  10  to  20  per  cent,  of  their  weight, 
the  kidneys,  blood,  and  muscles  between  20  to  30,  and  the 
liver  and  spleen  between  50  to  70. 

The  rate  of  waste  during  a  fast  depends  upon  the  amount 
of  energy  required,  and  it  is  therefore  increased  by  muscular 
work  and  by  exposure  to  cold.  When  a  man  is  kept  quiet 


374  HUMAN  PHYSIOLOGY 

and  warm  and  supplied  with  water,  a  fast  of  thirty  days  may 
in  some  cases  be  borne  without  injury. 

3.  Effect  of  Feeding. 

When  food  is  given  to  a  fasting  animal  or  man,  the  first 
effect  is  to  increase  the  rate  of  wasting  by  calling  into  action 
the  muscles  and  glands  concerned  in  digestion.  The  result 
is  an  immediate  increase  in  the  excretion  of  nitrogen  and 
carbon,  indicating  an  increased  breaking  down  of  proteids 
and  fats.  Zuntz  and  Magnus  Levy  found  that  a  diet  of  white 
bread  and  butter  increased  the  metabolic  processes  by  an 
amount  equivalent  to  about  10  per  cent,  of  the  energy  value 
of  the  diet.  For  this  reason,  to  give  an  animal  which  is  fast- 
ing a  diet  containing  just  the  amount  of  nitrogen  and  of 
carbon  which  the  animal  is  excreting,  will  not  at  once  stop 
the  loss  of  weight.  Suppose,  for  instance,  that  to  a  fasting 
animal  using  daily  30  grms.  of  the  proteids  and  160  grms.  of 
the  fats  of  his  body,  a  diet  containing  these  amounts  is  given, 
the  disintegration  of  proteids  and  of  fats  will  at  once  rise,  say, 
to  50  grms.  of  proteid  and  280  of  fat.  Thus  the  result  will 
be  that,  instead  of  his  losing  30  grms.  of  proteid,  he  will  lose 
only  20  grms.  per  diem,  and  instead  of  160  grms.  of  fat,  only 
120  grms.  But,  if  the  diet  is  sufficient  to  supply  the  energy 
required,  in  a  few  days  the  intake  and  output  will  balance, 
and  the  individual  is  then  said  to  be  in  metabolic  equili- 
brium, and  he  neither  gains  nor  loses  weight.  The  following 
table  gives  an  idea  of  how  this  adjustment  of  the  metabolism 
is  reached : — 


Day. 

Intake. 

Disintegrated. 

Waste  diminished  to 

Proteid. 

Fat. 

Proteid. 

Fat. 

Proteid. 

Fat. 

1 

0 

0 

30 

160 

2 

30 

160 

50 

280 

20 

120 

3              30 

160 

40 

240 

10 

60 

4 

30 

160 

30 

200 

0 

40 

5 

30 

160 

30 

160 

0 

0 

6              30 

160 

30 

160 

0 

0 

If  the  amount  of  food  be  further  increased,  a  small  pro- 
portion of  the  proteids  and  a  larger  proportion  of  the  fats  are 


THE   FOOD  AND  DIGESTION  375 

retained,  and  weight  is  gained.     As  already  indicated,  the 
power  of  storing  proteids  is  generally  small. 


Proteid  Diet. — Proteids  contain  all  the  chemical  elements 
required  for  the  building  and  repair  of  the  tissues,  and  from 
the  complexity  of  their  molecules  they  also  supply  latent 
energy.  It  is  therefore  theoretically  possible  for  an  animal 
to  sustain  life  on  proteids,  and  certain  animals  can  be  fed 
exclusively  upon  them.  Thus  Pfliiger  kept  a  dog  for  many 
months  upon  a  purely  proteid  diet  without  injury  to  its 
health.  But  to  supply  the  necessary  energy  in  proteids 
alone  requires  the  consumption  of  excessively  large  quanti- 
ties. For  a  man  to  get  the  energy  equivalent  to  3000  Calories 
— a  very  moderate  expenditure  per  diem — he  would  have  to 
eat  more  than  seven  times  the  usual  amount  of  proteid. 
Further,  it  has  been  shown  that,  when  large  quantities  are 
taken,  a  portion  is  broken  up  in  the  intestinal  wall  and 
formed  into  urea  by  the  liver  and  excreted  by  the  kidney, 
and  thus  excessive  work  is  thrown  upon  these  excretory 
organs.  While  these  organs  usually  form  and  excrete 
about  33  grms.  of  urea  per  diem,  on  such  a  diet  they  would 
have  to  deal  with  no  less  than  231  grms. 

It  is  therefore  not  advantageous  to  adopt  a  too  purely5 
proteid  diet.  The  great  use  of  proteids  is  as  muscle-builders. 
When  the  muscles  are  in  a  state  of  constant  activity  they 
have  a  certain  power  of  laying  on  proteid  as  they  grow. 
Hence  the  value  of  proteids  in  muscular  training. 

Gelatin,  although  undergoing  digestion  and  absorption  like 
the  proteids,  is  not  available  as  a  muscle-builder.  Its  sole 
use  is  as  an  energy  yielder,  and  in  this  respect  it  has  a  value 
equal  to  the  proteids. 

Carbohydrate  Diet. — Carbohydrates  are  of  equal  value 
with  proteids  as  a  source  of  energy,  but  they  contain  no 
nitrogen,  and  they  are  not  available  for  building  up  and 
repairing  the  protoplasm  of  muscles  and  other  tissues.  Car- 
bohydrates alone  will  not  support  life,  but  when  added  to 
proteids  they  enable  the  animal  to  do  with  smaller  quantities 
of  the  latter.  They  are  thus  sometimes  termed  proteid 


376  HUMAN   PHYSIOLOGY 

sparers.     Their  use  in  diminishing  the  consumption  of  pro- 
teids  is,  however,  strictly  limited. 

Fat  Diet. — Fats,  like  carbohydrates,  will  not  support  life, 
because  they  cannot  be  used  for  building  up  protoplasm,  but, 
like  carbohydrates,  they  are  a  source  of  energy,  and  they  have 
more  than  twice  the  energy  value  of  proteids  or  of  carbo- 
hydrates (p.  315).  They  are  thus  proteid  sparers.  But 
experiment  has  shown  that,  in  spite  of  their  higher  energy 
value,  they  have  not  the  same  power  as  carbohydrates  of 
sparing  proteids,  since  greater  work  is  required  in  their 
digestion  and  absorption. 

A  knowledge  of  the  part  played  by  proteids,  carbohydrates, 
and  fats  in  the  animal  body  is  the  groundwork  of  the  study 
of  Dietetics. 

DIETETICS. 

The  great  essentials  of  a  diet  capable  of  maintaining  health 
are : — 

1st.  That  it  should  supply  the  energy  required. 

2nd.  That  it  should  contain  sufficient  proteids  to  make 
good  the  waste  of  these  substances. 

3rd.  That  it  should  be  capable  of  digestion,  absorption, 
and  assimilation. 

I.  The  energy  requirements  vary  with  the  mode  of  life  and 
with  the  age  and  size  of  the  individual. 

Size. — Other  things  being  equal,  a  large  man  requires  more 
energy  and  more  proteid  than  a  small  man.  For  this  reason 
the  energy  requirements  are  sometimes  stated  as  per  unit  of 
weight,  but  it  is  more  convenient  to  take  as  the  standard  an 
adult  man  of  average  weight,  say  65  kgs.  It  must  further 
be  remembered  that  the  smaller  the  animal  the  greater  the 
surface  in  proportion  to  its  weight ;  and  hence  the  greater 
the  loss  of  heat  per  unit  of  weight.  For  this  reason  alone 
small  animals  and  children  require  more  energy  per  unit  of 
weight  than  larger  animals  or  older  people. 

Age. — In  children  the  metabolism  is  more  active  than  in 
adults.  They  are  more  constantly  in  motion,  and  they  re- 
quire energy  and  material  to  build  up  their  tissues,  and,  as 


THE   FOOD   AND   DIGESTION 


377 


just  stated,  the  loss  of  heat  per  unit  of  weight  is  greater  than 
in  their  elders.  Weight  for  weight,  a  child  thus  requires 
a  greater  supply  of  energy  than  a  man.  The  following 
results,  based  upon  observations  on  diets  recorded  by 
Camerer,  illustrate  this  : — 


Age. 

Weight  in  Kilos. 

Energy  used  per  Kilo 
in  Calories. 

Total  Energy 
in  Calories. 

4 
12 
30 

14 
30 
66 

91-3 

57-7 
42-4 

1280 
1730 
2800 

It  will  thus  be  seen  that  the  energy  requirements  of  chil- 
dren at  different  ages  may  be  stated  in  terms  of  the  energy 
requirements  of  an  adult  man  doing  average  work.  Atwater 
formulates  this  as  follows  : — 


Taking  a  man  at 
A  woman 
A  boy  of  14  to  16 
A  girl 

A  child  10  to  13 

6  to    9 

2  to    5 

under  2 


.     1-0 

is  equivalent  to  0-8  of  a  man. 
0-8         „ 
0-7 

0-6         „ 
0-5         „ 
0-4 
0-3 


Mode  of  Life. — An  individual  kept  warm  and  at  rest,  re- 
quires much  less  energy  than  if  he  is  required  to  do  muscular 
work  and  is  exposed  to  cold.  The  variations  in  the  amount 
of  material  used  in  different  conditions  is  well  illustrated  by 
the  experiments  of  Zuntz  on  the  excretion  of  carbon  dioxide 
in  the  dog. 


Resting  lying 

„       standing      .... 
Forward  movement  (unloaded) 

(drawing  weight) 


CO2  excreted  per 
minute  in  corns. 

124-7 
170-2 
525-0 
798-9 


These  results  are  corroborated  by  a  study  of  the  diet  re- 
quired to  maintain  the  weight  and  health  of  men  doing 
different  amounts  of  work.  From  a  large  series  of  such 


378  HUMAN   PHYSIOLOGY 

observations  Atwater  concludes  that  the  energy  requirements 
of  the  diet  varies  as  follows  : — 

Calories. 

Man  without  muscular  work  .  2700 

„     with  light  muscular  work       .  3000 

„        „     moderate  muscular  work  3500 

„        „    severe  muscular  work     .  4500 

It  must,  of  course,  be  remembered  that  all  the  energy  of 
the  constituents  of  the  diet  is  not  available,  since  a  consider- 
able and  varying  proportion  of  the  food  is  not  digested  or 
absorbed  (p.  381).  The  gross  energy  of  the  diet  must  there- 
fore be  well  above  the  nett  energy  requirements  of  the  body. 

A  rough  idea  of  these  nett  requirements  may  be  arrived  at 
by  considering  the  average  expenditure  of  energy  in  different 
forms.  Taking  the  case  of  a  man  doing  a  moderate  day's 
muscular  work  equivalent  to  150,000  kgms.  or  350  Calories 
(see  p.  73),  he  gives  off,  according  to  N.  Stewart's  calcula- 
tions (Manual  of  Physiology,  p.  425),  2590  Calories  of  heat. 
The  loss  of  energy  is  thus — 

Calories. 

In  external  muscular  work,  equivalent  to       350 
As  heat 2600 

Total    .  .  .  2950 

To  make  good  this  loss  of  energy  the  diet  should  contain 
more  than  3000  gross  Calories,  and  the  results  of  Zuntz 
clearly  show  that,  when  muscular  work  is  large  in  amount, 
a  greater  supply  of  energy  must  be  forthcoming,  while  in 
individuals  leading  a  sedentary  life  the  energy  requirement 
will  be  smaller.  In  the  case  of  a  labourer  it  is  safe  to  allow 
at  least  3500  Calories. 

II.  Proteid  Requirements. — Many  experiments  have  been 
made  to  determine  the  smallest  amount  of  proteids  upon 
which  life  can  be  maintained,  and  conflicting  results  have 
been  arrived  at.  Various  investigators  have  succeeded  in 
maintaining  a  nitrogenous  equilibrium  for  short  periods  on 
an  intake  of  proteid  no  greater  than  that  metabolised  in 
fasting. 


THE   FOOD   AND  DIGESTION  379 

Recently  Chittenden  has  recorded  a  prolonged  series  of 
investigations  upon  this  question  by  which  he  shows  that  in 
five  professional  men  for  periods  of  from  seven  to  nine 
months,  in  eight  student  athletes  for  from  four  to  seven 
months,  and  in  thirteen  soldiers  for  five  months,  a  nitro- 
genous equilibrium  and  perfect  physical  and  mental  health 
could  be  maintained  with  an  intake  of  from  35  to  56  grms. 
of  proteid  per  diern. 

The  work  done  by  these  individuals  is  not  recorded,  but 
from  the  daily  routine  of  the  soldiers,  whose  chief  work 
appears  to  have  been  two  hours'  exercise  in  a  gymnasium,  it 
must  be  classed  as  very  moderate.  The  fact  that  the  energy 
value  of  the  diets  in  the  case  of  the  soldiers  was  only  2500  to 
2800  Calories  seems  either  to  confirm  the  opinion  that  they 
were  not  called  upon  to  do  severe  work  or  to  suggest  that  our 
present  estimates  of  the  loss  of  energy  as  heat  from  the  body 
(p.  378)  are  erroneous. 

While  these  results  certainly  prove  that  men  can  maintain 
health  and  muscular  efficiency  for  long  periods  on  about  half 
the  amount  of  proteid  which  is  usually  consumed,  they  do 
not  demonstrate  that,  in  the  case  of  those  subjected  to 
strenuous  and  sustained  muscular  work,  such  a  reduction  is 
desirable,  nor  do  they  indicate  that  in  growing  children  a 
reduction  in  the  amount  of  proteids  usually  consumed  may 
be  safely  allowed.  A  study  of  a  very  large  series  of  dietaries 
of  different  races  shows  that,  unless  absolutely  prevented  by 
poverty,  the  average  man  consumes  over  100  grms.  of  pro- 
teid per  diem,  and  that  those  in  muscular  training  tend  to 
consume  very  much  larger  amounts.  Bearing  in  mind  the 
importance  of  proteids  as  muscle  builders,  it  is  safe  to  con- 
clude that  about  120  grms.  of  proteids  should  be  allowed  per 
man  per  diem,  at  least  in  the  labouring  classes. 

Ill;  Part  played  by  Carbohydrates  and  Fats. — The  carbo- 
hydrates and  fats  have  to  supply  the  energy  not  supplied  by 
the  proteids.  The  amounts  which  must  be  consumed  will 
thus  depend,  first,  on  the  amount  of  proteid  taken,  and, 
second,  on  the  energy  requirement  of  the  individual.  If  100 
grms.  of  proteid  are  taken,  this  will  yield  410  Calories  of 
energy,  while  if  only  50  are  consumed,  205  Calories  will  be 


38o  HUMAN  PHYSIOLOGY 

yielded.  The  difference  between  these  amounts  and  the  total 
energy  requirements  of  the  person  must  be  made  up  from 
carbohydrates  and  fats.  In  the  case  of  a  working-man 
requiring  3500  Calories,  and  consuming  100  grms.  of  proteid 
per  diem,  this  leaves  about  3000  Calories  to  be  supplied  by 
fats  and  carbohydrates. 

In  determining  the  proportionate  amounts  of  these,  two 
factors  have  to  be  considered — first,  the  relative  cost,  and, 
second,  the  limitation  of  the  power  of  digestion  of  each. 

Carbohydrates  are  enormously  cheaper  than  fats  as  a 
source  of  energy.  Margarine  at  8d.  per  Ib.  will  yield  435 
Calories  for  a  penny,  while  sugar  at  2|d.  per  Ib.  will  yield 
1860  Calories  for  the  same  sum.  But  the  use  of  carbo- 
hydrates as  a  source  of  energy  is  limited  by  the  fact  that  in 
most  individuals  digestive  disturbances  are  apt  to  supervene 
if  more  than  from  500  to  600  grms.  are  consumed. 

A  diet  should  therefore  not  contain  much  more  than  500 
grms.  of  carbohydrates,  and  these  will  yield  2050  Calories  of 
energy. 

This  leaves  about  950  Calories  to  be  supplied  by  fats,  an 
amount  which  is  nearly  met  by  100  grms.  of  fat  yielding  930 
Calories  of  energy.  This  quantity  of  fat  most  people  can 
easily  digest. 

A  typical  diet  for  an  average  man  doing  moderate  muscular 
work  would  thus  be — 

Amount.  Energy  Value  in 

Calories. 

Proteids       ...       120  410 

Carbohydrates     .         .       500  2050 

Fats  100  930 


3390 

or  very  nearly  the  3500  Calories. 

A  reference  to  Atwater's  table  gives  the  dietary  require- 
ments of  a  woman  or  of  a  child  of  any  age,  and  thus  renders 
it  easy  to  make  out  the  diet  of  a  family  or  public  institution. 

IY.  The  Diet  must  be  capable  of  Digestion,  Absorption, 
and  Assimilation. — While  proteids,  carbohydrates,  and  fats 
in  the  alimentary  canal  undergo  the  changes  already  de- 


THE  FOOD   AND   DIGESTION  381 

scribed,  they  are  frequently,  when  taken  in  the  food,  in  an 
unfavourable  state  for  the  action  of  digestive  juices.  Thus, 
while  the  proteids  of  flesh  are  exposed  to  the  gastric  and 
pancreatic  secretion,  the  proteids  of  many  vegetables,  unless 
carefully  prepared,  cooked,  and  masticated,  are  protected  and 
escape  digestion  and  absorption.  The  same  may  be  said  of 
the  crude  starch  of  vegetables.  The  digestibility  and  value 
as  an  article  of  diet  of  the  cellulose  and  allied  substances  in 
plants  is  an  important  question  at  present  requiring  further 
investigation. 

The  fats  of  vegetables  are  also  generally  in  a  less  favourable 
state  for  digestion  than  the  fats  of  animals. 

When  once  digested  there  seems  to  be  no  difference  in  the 
absorbability  and  assimilability  of  the  proximate  principles 
of  vegetables  and  animals ;  a  given  weight  of  vegetable  pro- 
teid  may  be  substituted  for  the  same  quantity  of  animal 
proteid. 

As  a  result  of  the  difference  in  digestibility  the  availa- 
bility of  food-stuffs  varies.  The  following  table  serves  to 
show  some  of  these  variations. 

Absorption  of  Food-stuffs. 

Per  Cent.  Absorbed. 
Proteid.  Fat.         Carbohydrate. 

Flesh           ...       97  95 

Egg     ....       97  95 

Milk    ....    89-99  96  100 

Bread           ...       78  ...  99 

Potatoes  (boiled)          .       68  ...  92 

Carrots  (raw)        .         .61  ...  82 

The  availability  of  almost  any  article  of  food  varies  with 
the  state  of  the  teeth  and  the  digestive  organs  of  the 
individual,  with  the  manner  in  which  it  is  eaten — whether 
leisurely  or  too  rapidly  and  without  proper  mastication,  and 
with  the  manner  in  which  it  is  prepared.  For  example,  the 
mode  of  manufacture  of  the  flour  used  in  bread-making 
has  a  very  marked  influence  upon  the  amount  digested  and 
absorbed.  In  vegetable  foods  especially  the  thoroughness 
of  the  cooking  has  a  most  important  influence  on  the  availa- 
bility of  their  constituents. 


382  HUMAN  PHYSIOLOGY 

By  assimilation  is  meant  the  taking  of  the  food  con- 
stituent from  the  blood  by  the  muscle  and  other  tissues  so 
that  it  may  be  used,  for  until  the  constituents  of  the  food 
have  become  part  of  the  protoplasm  of  the  tissues  they 
are  not  oxidised.  Apparently  the  only  substances  which 
can  be  freely  assimilated  and  used  by  the  tissues  are  the 
three  proximate  principles  of  the  food,  although  alcohol  and 
possibly  some  other  similar  substances  may  be  utilised  to  a 
small  extent. 

Y.  The  subject  of  Dietetics  cannot  be  left  without  allud- 
ing to  the  influence  of  two  very  universally  used  materials — 
alcohol  and  tea. 

Alcohol. — The  influence  of  alcohol  may  be  considered 
under  two  heads. 

1st.  Before  absorption.  Moderate  doses  of  alcohol  taken 
along  with  food  do  not  appear  to  interfere  with  the  digestion 
and  absorption  of  proteids,  of  carbohydrates,  or  of  fats. 
When  taken  apart  from  food  and  in  such  excessive  doses 
as  to  act  as  an  irritant  and  to  cause  catarrh  of  the 
alimentary  canal,  alcohol  of  course  has  a  prejudicial 
influence. 

2nd.  After  absorption.  The  tissues  have  the  power  of 
oxidising  a  small  quantity  of  alcohol,  and  since  the  com- 
bustion of  1  grm.  of  alcohol  liberates  7'06  Calories  of  energy, 
alcohol  must  be  regarded  as  a  food  just  in  the  same  way  as 
sugar  is  a  food.  The  amount  of  alcohol  which  can  be  thus 
oxidised  varies  in  different  individuals,  but  the  average 
man  cannot  use  more  than  50  grms.  per  diem.  Any 
alcohol  taken  above  the  amount  which  can  be  oxidised  is 
excreted  in  the  urine  and  breath,  and  in  its  passage  through 
the  body  it  acts  as  a  poison  to  the  protoplasm,  first  diminish- 
ing its  activity  and  so  diminishing  the  rate  of  waste,  and 
secondly  causing  the  death  of  the  protoplasm  and  the 
removal  of  the  broken-down  nitrogenous  constituents  in  the 
urine.  Its  poisonous  action  on  the  liver  is  demonstrated 
by  the  diminished  building  up  of  urea,  so  that  a  greater 
quantity  of  waste  nitrogen  is  excreted  in  other  forms.  In 
moderate  doses  it  stimulates  the  heart  and  dilates  the 
arterioles.  Its  first  action  in  checking  the  activity  of  meta- 


THE   FOOD    AND   DIGESTION  383 

holism,  along  with  its  influence  in  producing  dilatation  of 
the  cutaneous  vessels,  is  taken  advantage  of  in  its  use  in 
the  treatment  of  high  temperature  in  some  fevers. 

Tea,  Coffee,  and  Cocoa  have  as  their  essential  constituents 
the  substances  theine  and  caffeine,  which  are  methyl  deriva- 
tives of  xanthin,  a  diureide  which  is  excreted  in  the  urine 
(see  p.  397).  They  tend  to  paralyse  the  sensory  mechanism 
of  the  brain,  and  hence  abolish  the  sense  of  fatigue.  They 
stimulate  the  heart  and  the  secreting  action  of  the  kidneys, 
but  they  have  no  effect  upon  the  metabolism.  Cocoa  nibs 
contain  about  8  per  cent,  of  proteid  and  about  50  per  cent, 
of  fat.  Cocoa  is  therefore  an  energy -yielding  food. 


SECTION  X 

INTERNAL  SECRETIONS— THEIR  PRODUCTION 
AND   ACTION 

THE  products  of  the  metabolism  of  the  various  organs 
are  carried  away  in  the  lymph  and  blood,  and  certain  of 
these  products  exercise  an  important  influence  upon  other 
structures  in  the  body.  These  products  have  been  termed 
Internal  Secretions,  and  the  mode  of  action  of  some  of 
them  has  been  more  or  less  investigated.  Much,  however, 
remains  to  be  done  before  our  knowledge  can  be  deemed 
anything  like  satisfactory. 

Among  the  structures  which  are  known  to  yield  such 
active  internal  secretions  are  the  following : — 

1.  Suprarenal  Bodies. — These  structures  lie  just  above 
the  kidneys.  Each  consists  of  a  tough  cortex  composed 
of  epithelial-like  cells  arranged  in  columns,  and  a  soft 
medulla  consisting  of  cells  derived  from  neuron  cells 
which  stain  of  a  peculiar  brown  colour  with  chromic  acid. 
The  medulla  is  developed  from  the  sympathetic  chain  of 
ganglia.  The  cortex  is  a  perfectly  independent  structure 
derived  from  the  surrounding  mesoblast,  and  in  teleostean 
fishes  it  is  quite  apart  from  the  representative  of  the  medul- 
lary part.  A  suprarenal  body  is  thus  two  distinct  and 
independent  organs  combined  with  one  another  (Fig.  151). 

Long  ago  Brown-Sequard  stated  that  removal  of  these 
bodies  causes  great  muscular  weakness,  loss  of  tone  of  the 
vascular  system,  loss  of  appetite,  and  finally  death  in  a  short 
time.  Addison  had  already  pointed  out  that  a  similar  set 
of  symptoms,  accompanied  by  pigmentation  of  the  skin,  is 
associated  with  diseased  conditions  of  these  organs  in  man. 
Within  the  last  few  years  it  has  been  demonstrated  that  in- 
jections of  small  quantities  of  the  medullary  portion  of  the 

bodies  have  a  powerful  effect  on  the  muscular  system  gene- 

384 


INTERNAL  SECRETIONS 


385 


rally,  and  especially  on  the  muscles  of  the  vascular  system, 
causing  contraction  of  the  arterioles,  and  an  enormous  rise 
in  the  blood-pressure.  It  seems  to  act  not  directly  on  the 
muscle  substance,  but  rather  through  the  sympathetic  nerve 
endings.  Thus,  McFie  got  no  action  on  the  heart  of  the  chick 
in  which  the  nerves  are  not  yet  developed,  while  Brodie  has 
failed  to  get  any  effect  on  the  vessels  of  the  lungs  in  which 
the  sympathetic  nerve  terminations  are  said  to  be  absent. 


FIG.  151. — Section  through  Cortex  and  Medulla  of  the  Suprarenal  Body  of 
a  mammal ;  a,  6,  c,  d,  Cortex ;  /,  Medulla. 


Apocodeine,  which  poisons  the  nerve  endings,  abolishes  the 
effect  of  suprarenal  extracts  on  the  blood  vessels. 

On  the  heart  it  has  an  inhibitory  action  through  the 
vagi,  and  this  may  be  removed  by  section  of  the  nerves  or 
by  the  administration  of  atropine,  which  poisons  their  termi- 
nations. 

As  already  indicated  (p.  368),  injections  of  extracts  of  the 
suprarenal  bodies  profoundly  modify  the  metabolism,  lead- 
ing to  an  increase  of  sugar  in  the  blood  and  to  its  excretion 
in  the  urine.  This  is  best  marked  when  the  animal  is  well 
fed  and  has  a  store  of  glycogen  in  its  liver,  but  since  it 
occurs  in  fasting  animals  after  the  stored  carbohydrates 

25 


386  HUMAN  PHYSIOLOGY 

have  been  cleared  out  by  the  administration  of  phloridzin 
(p.  368),  it  would  appear  to  be  due,  in  part  at  least,  either 
to  a  non-utilisation  of  sugar  by  the  tissues  or  to  an  increased 
production  of  sugar  from  proteids.  A  decrease  in  the  nitrogen 
in  the  form  of  urea  and  an  increase  of  that  in  ammonia, 
similar  to  that  found  in  cases  of  true  diabetes,  have  also 
been  observed. 

It  has  been  suggested  that  the  suprarenal  secretion  acts 
through  the  pancreas  by  preventing  the  formation  of  the 
internal  secretion  which  has  been  supposed  to  act  on  the 
liver  (see  p.  390).  But  in  birds  it  acts  even  after  removal 
of  the  pancreas. 

The  essential  principle  of  the  suprarenals  is  a  substance, 
adrenalin,  the  constitution  of  which  is  now  known.  Various 
more  or  less  successful  attempts  have  been  made  to  prepare 
it  synthetically. 

2.  Pituitary   Body. — This   lies   at  the  base   of  the  mid- 
brain,  and  consists  of  a  posterior   part  of  nervous    tissue, 
somewhat  resembling  the  medulla  of  the  suprarenals,  and 
an  anterior  part  derived  from  the  alimentary  canal,   and 
consisting  of  masses  of  epithelial-like  cells. 

Removal  of  this  body  causes  in  cats  and  dogs  a  fall  of 
temperature,  lassitude,  muscular  twitchings,  dyspmjea,  and 
ultimately  death.  Injection  of  extracts  of  the  substance  is 
said  to  diminish  these  symptoms.  In  the  healthy  animal 
the  injection  of  extracts  of  the  posterior  or  nerve  parts  of 
the  pituitary  causes  an  augmentation  of  the  force  of  cardiac 
contraction,  and  a  contraction  of  the  arterioles,  and  thus 
raises  the  arterial  pressure. 

3.  Thyroid  Gland  (Fig.  152). — This  structure  is  formed  as  a 
hollow  outgrowth  for  the  anterior  part  of  the  alimentary  canal, 
which  branches  and  again  branches.     It  early  loses  its  con- 
nection with  the  alimentary  canal,  and  becomes  cut  up  by 
fibrous  tissue  into  a  number  of  small  more  or  less  rounded 
cysts  or  follicles,  each  lined  with  epithelium,  and  filled  with 
a  mucus-like  substance,  which  contains  a  nucleo-proteid,  and 
a  substance  with  a  marked  power  of  combining  with  iodine. 
This  has  been  called  lodothyrin.     It  contains  about  3'6  per 
cent,  of  iodine,  and  it  seems  to  be  the  active  constituents  of 
the  internal  secretion  of  the  gland. 


INTERNAL  SECRETIONS 


387 


The  removal  of  the  thyroid  usually  leads  to  a  train  of 
symptoms  which  varies  somewhat  in  different  animals,  but  is 
essentially  the  same  in  nearly  all.  The  connective  tissues 
tend  to  revert  to  the  embryonic  conditions,  and  the  amount 
of  mucin  increases.  The  temperature  falls,  muscular  tremors 
appear,  and  in  dogs  these  may  go  on  to  convulsions.  They 
do  not  disappear  on  removing  the  cortex  cerebri,  but  are 
stopped  by  section  of  the  nerves  to  the  muscles,  and 
thus  appear  to  be  spinal  in  origin.  The  function  of  the 


TV, 


FIG.  152.— Section  through  part  of  the  Thyroid  (Th.)  and  a  Parathyroid  (P.) 
of  a  mammal. 


higher  nervous  system  becomes  sluggish,  and  the  animal 
usually  dies.  By  administering  the  substance  of  the 
thyroid,  or  by  giving  extracts  of  the  thyroid,  most  of  these 
symptoms  may  be  delayed  or  prevented.  When  thyroid 
gland  or  extract  is  given  to  healthy  animals  in  moderate 
doses  it  causes  an  increased  metabolism  of  both  fats  and 
proteids,  and  may  thus  induce  emaciation.  It  would  appear 
as  if  one  function  of  the  organ  is  to  produce  an  internal 
secretion,  which  regulates  the  rate  of  the  metabolic  processes 
in  the  body  by  increasing  them  when  such  an  increase  is 
desirable.  It  seems  also  to  act  slightly  on  the  arterioles  as  a 


388  HUMAN   PHYSIOLOGY 

vaso-dilator.  When  the  thyroid  is  not  developed,  the  growth 
and  development  of  the  individual  are  partially  arrested, 
and  the  condition  of  cretinism  is  produced.  Atrophy  of  the 
structure  in  adult  life  causes  a  train  of  symptoms  somewhat 
resembling  those  produced  by  its  removal,  and  constituting  the 
disease,  myxcedema.  It  has  been  suggested  that  a  condition 
of  increased  activity  of  the  action  of  the  heart,  usually  accom- 
panied by  prominence  of  the  eyes  and  swelling  in  the  region 
of  the  thyroid — exophthalmic  goitre — may  be  due  either  to 
increased  activity  of  the  structure,  or  to  deficient  action  of 
the  parathyroids. 

4.  Parathyroids. — Two  to  four  small  nodules  are  found  in 
close  relationship  to  each  lobe  of  the  thyroid  often  lying  in 
its  substance,  and  these  are  formed  of  columns  of  cells  with 
capillary  blood  vessels  between  them  (Fig.  152).     More  or  less 
successful  attempts  have  been  made  in  different  animals  to 
remove  them  without  the  thyroid,  or  the  thyroid  without 
them,  and  the  general  result  of  these  experiments  is  that  the 
nervous  symptoms  which  follow  ordinary  thyroidectomy — the 
tremors,  &c. — seem  to  be  due  to  the  loss  of  the  parathyroids, 
while  the  metabolic  changes  are  probably  due  to  want  of  the 
internal  secretion  of  the  thyroid. 

5.  Ovaries  and  Testes.  —  It  is  well  known  that  removal  of 
these  organs  causes  characteristic  changes  in  the  animal ;   a 
tendency  to  the  deposition  of  fat  being  produced,  the  activity 
of  the  central  nervous  system  being  somewhat  modified,  the 
voice  in  the  male  losing  its  masculine  character,  and  the 
thymus  persisting,  in  the  male  at  least,  for  a  considerably 
longer  period  than  in  the  normal.     Several  years  ago  Brown- 
Sequard  maintained  that  by  the  administration  of  testicular 
substance  the  general  effects  of  atrophy  of  these  organs  might 
be  obviated  ;  and  more  recently,  as  a  result  of  clinical  experi- 
ence, the  administration  of  extracts  of  the  ovaries  has  been 
described    as    relieving   certain   of  the  nervous  symptoms 
which  supervene  on  their  removal  or  atrophy.    It  has  further 
been  found   that   ovarian   substance  when   given   to   dogs, 
whether  male  or  female,  causes  an  increase  in  the  rate  of 
proteid  metabolism,   although   no   similar  action   is   found 
with  testicular  substance.      There  is  thus  evidence  that  the 
ovaries,  like  the  thyroid,  form  an  internal  secretion  having 


INTERNAL   SECRETIONS 


389 


an  important  action  in  accelerating  the  metabolism,  and 
it  is  at  least  probable  that  the  testes  produce  a  similar 
substance. 

6.  Thymus.  —  This  structure  develops  as  an  epithelial 
outgrowth  from  one  or  more  of  the  branchial  arches  of  the 
embryo.  Round  these  outgrowths  masses  of  lymph'  tissue 


FIG.  153. — Section  of  the  lobules  of  the  Thymus  to  show  the  lobules, 
with  Hassall's  corpuscles  in  the  central  part. 

collect,  and  thus  a  much  lobulated  structure  lying  in  the  front 
of  the  neck  and  upper  part  of  the  thorax  is  formed.  As  de- 
velopment goes  on  the  epithelial  core  of  each  lobule  breaks 
up  and  forms  nests  of  cells  which  are  often  disposed  concen- 
trically and  form  the  corpuscles  of  Hassall.  These  lie  in  a 
loose  lymphoid  tissue,  the  medulla  of  the  lobule,  and  this 
is  surrounded  by  a  cortex  of  dense  lymphoid  tissue.  The 
thymus  is  largest  in  relationship  to  the  body  weight  about 
the  time  of  birth,  but  it  continues  to  grow,  although  not  in 
proportion  to  the  growth  of  the  body,  till  about  the  age  of 
puberty.  After  about  twenty-four  years  of  age  it  atrophies 
and  is  replaced  by  a  mass  of  fatty  tissue.  The  Hassall's  cor- 
puscles seein  to  atrophy  earlier  than  the  lymphoid  tissue. 
Castration  in  cattle  and  guinea-pigs  markedly  retards  the 


390  HUMAN   PHYSIOLOGY 

onset  of  atrophy,  so  that  the  thymus  of  the  ox  is  much 
larger  than  that  of  the  bull  of  the  same  age.  Not  only  so, 
but  removal  of  the  thymus  in  young  guinea-pigs  seems  to  be 
followed  by  a  more  rapid  growth  of  the  testes.  It  is  therefore 
probable  that  the  thymus  yields  an  internal  secretion  which 
controls  the  growth  of  the  testes. 

The  only  other  effect  of  its  removal  in  young  guinea-pigs 
is  a  diminution  in  the  number  of  leucocytes.  This  seems  to 
lead  to  a  diminished  power  of  resisting  the  invasion  of  those 
micro-organisms  —  e.g.  staphylococci  —  which  are  normally 
combated  by  the  leucocytes. 

7.  Pancreas. — That-  the  pancreas  is  the  most  important  of 
the  digestive  glands  has  been  for  long  known,  but  a  further 
function  has  more  recently  been  demonstrated.  It  has  been 
found  that  the  excision  of  the  pancreas  in  dogs  and  other 
mammals  produces  a  condition  of  diabetes — an  increase  of 
sugar  in  the  blood,  its  appearance  in  the  urine,  an  increased 
excretion  of  nitrogen,  and  a  general  emaciation.  In  ducks 
and  geese  this  effect  is  not  produced.  These  symptoms  do 
not  occur  when  the  duct  is  tied  or  occluded  until  degenera- 
tion has  developed,  but  they  are  invariable  and  immediate 
when  a  sufficient  amount  of  the  gland  is  removed.  They 
are  not  prevented  by  the  administration  of  pancreas,  either 
fresh  or  as  extracts.  In  this  condition,  sugar  is  formed 
from  the  proteids,  since  it  appears  after  all  the  glycogen  has 
been  removed,  and  its  amount  is  proportionate  to  the  amount 
of  nitrogen  excreted.  The  pancreas  seems  therefore  to  form 
something  which  either  controls  the  production  of  sugar  in 
the  liver  or  causes  its  utilisation  by  the  muscles.  Since  the 
only  respect  in  which  the  pancreas  differs  in  histological 
character  from  the  parotid  gland — removal  of  which  has  no 
effect  on  the  metabolism — is  in  the  presence  of  the  islets  of 
Langerhans,  it  has  been  suggested  that  they  are  related  to  this 
function  of  the  organ.  Some  recent  observations,  however, 
tend  to  show  that  these  islets  are  not  permanent  structures, 
but  that  they  are  formed  from  and  revert  to  the  ordinary 
secreting  tissue.  The  fact  that,  in  ducks  and  geese  in 
which  removal  of  the  pancreas  does  not  cause  diabetes, 
the  injection  of  suprarenal  extracts  causes  glycosuria,  is 
opposed  to  the  view  that  it  acts  through  the  pancreas, 


INTERNAL  SECRETIONS  391 

and  suggests  that  it  must  act  directly  on  the  liver  or  other 
tissues. 

8.  Duodenum. — As  already  pointed  out  (p.  349),  the  duo- 
denum yields  an  internal  secretion  (secretin),  which  acts 
directly  upon  the  pancreas  to  stimulate  its  secretion. 

Toxic  Action  and  Immunity. 

It  is  not  definitely  known  how  these  internal  secretions 
each  perform  its  special  action,  but  light  seems  to  be  thrown 
upon  the  question  by  the  study  of  the  mode  of  action  of 
various  toxic  substances,  and  the  mode  of  production  of  a 
condition  of  immunity  against  them.  As  will  be  presently 
shown,  a  process  of  the  same  nature  as  the  production  of 
internal  secretions  is  involved. 

Snake  and  Diphtheria  Toxins. — It  may  be  most  simply 
explained  by  considering  first  the  probable  mode  of  action 
of  the  toxin  or  poison  of  snake  venom,  or  of  that  produced 
by  the  diphtheria  bacillus,  and  the  way  in  which  protection 
against  these  is  established  by  the  development  of  anti- 
toxins. 

By  injecting  under  the  skin  of  the  horse  increasing 
doses  of  such  toxins  the  animal  becomes  quite  resistant 
to  the  poison,  and  if  now  some  of  the  horse's  serum  is 
taken  it  is  found  that  a  certain  quantity  can  neutralise  a 
definite  quantity  of  the  toxin,  so  that  if  the  mixture  is  in- 
jected it  does  no  harm  to  the  animal.  Something  has  been 
formed  in  the  horse  which  seizes  on  the  molecules  of  the 
toxin  and  makes  them  harmless,  just  as  when  soda  is  added 
to  sulphuric  acid  it  forms  a  neutral  salt. 

The  two  molecules  have  a  definite  chemical  affinity  for  one 
another,  so  that  the  toxin  is  no  longer  free  to  seize  upon  the 
protoplasm  of  the  animal's  body.  To  explain  this  Ehrlich 
has  suggested  that  the  protoplasm  molecule,  like  the  proteid 
molecule  (p.  10),  is  to  be  considered  as  made  up  of  a  central 
core  with  a  number  of  side  chains  or  hands  which  play  an 
important  part  in  taking  up  nourishment  of  different  kinds, 
for  each  variety  of  which  special  side  chains  have  a  special 
affinity  (Fig.  154).  He  supposes  that  some  of  these  side  chains 
fit  the  toxin  molecule,  and  are  thus  capable  of  anchoring  it 


392  HUMAN  PHYSIOLOGY 

to  the  cell  and  allowing  it  to  exercise  its  toxic  action,  and  he 
explains  the  production  of  antitoxin  by  supposing  that,  as 
these  side  chains  get  linked  to  the  toxin  and  are  thus  as  it 
were  thrown  out  of  action,  others  are  produced  to  take  their 
place,  since  they  are  necessary  for  the  nourishment  of  the 
protoplasm.  If  the  toxin  is  continually  administered  in 
small  doses  this  production  of  side  chains  may  be  so  in- 
creased that  they  get  thrown  off  into  the  blood  and  in  it 
are  capable  of  linking  to  the  toxin  and  so  preventing  it 
from  fixing  itself  to  the  cells.  If  therefore  some  of  the 
blood  is  injected  into  an  animal  which  afterwards  receives 
a  dose  of  the  toxin,  that  toxin  will  not  act,  and  the  animal 

will  be  immune. 

Typhoid  Toxin. — But  immu- 
nity may  also  be  established  not 
against  toxins  separate  from 
organisms,  but  against  organisms 
which  hold  their  toxin,  as  in  the 
case  of  the  bacillus  of  typhoid 
fever.  Here  repeated  injections 

FIG  154.-TO  illustmte  the  forma-       of     increasing     doses     produce    a 
tion  of  Side  Cha:ns,   SC,   by  .     °   . 

which  the  toxin  molecules,  T,     serum  which   has  the  power  of 

are  either  anchored  to  the  cell       destroying     the     Organism     when 

:Lr±tt7"an?-      added    to  it  even    outside    the 
toxin  is  formed.  body.     But  this  is  not  a  simple 

combination,  because  if  the  serum 

be  heated  to  55°  C.  it  loses  its  power,  but  if  a  few  drops 
of  the  fresh  serum  of  an  unimmunised  animal  be  added, 
the  power  is  restored.  Obviously  the  anti-body  which 
destroys  the  organism — the  bacteriocidal  or  bacteriolytic 
body — requires  the  co-operation  of  another  body  to  enable 
it  to  act,  and  this  body  has  been  called  the  complement. 
Ehrlich  supposes  that  the  immune  body  does  link  to  the 
protoplasm  of  the  organism,  but  that  it  must  in  its  turn  be 
linked  to  the  complement.  The  figure  may  help  to  explain 
this  (Fig.  155). 

Cytotoxins. — Similar  anti-bodies,  acting  upon  the  cells 
of  the  animal  body,  may  be  produced  by  injecting 
the  particular  kind  of  cell  into  an  animal  of  another 
species.  Thus,  if  human  blood  be  repeatedly  injected 


INTERNAL  SECRETIONS 


393 


into  a    rabbit   the   serum   of   the   rabbit's   blood   becomes 
hwnolytic  —  i.e.    acquires    the    power    of    dissolving    the 


com 


a 


erythrocytes  in  human  blood.  In 
this  case  too  the  immune  body  re- 
quires the  presence  of  a  comple- 
ment, readily  destroyed  at  a  low 
temperature,  to  enable  it  to  act. 

Precipitins.  —  By  the  injection  of 
the  proteids  of  the  blood  of  any  parti- 
cular animal  into  an  animal  of  another 
species  a  serum  is  developed  which 


precipitates  the  proteids  of  the  blood     FIG.  155.— To  illustrate  the 


of  the  first  species  and  of  no  others. 

In  these  various  cases  the  active 
body  is  produced  by  the  throwing 
off  of  side  chains  from  protoplasm, 
and  as  these  products  are  carried  away  in  the  blood  the 
process  is  exactly  analogous  to  the  formation  of  internal 
secretions. 


anchoring  of  the  anti- 
body, a,  to  the  cell  by 
a  side  chain,  sc,  and 
the  action  upon  it  of 
complement,  com. 


SECTION  XI 
EXCRETION    OF    MATTER    FROM    THE    BODY 

1.  EXCRETION  BY  THE  LUNGS  (see  Respiration,  p.  277) 
2.  EXCRETION  BY  THE  KIDNEYS 

URINE 

THE  water  and  waste  nitrogen  of  the  body  are  chiefly  elimi- 
nated in  the  urine,  which  is  secreted  by  the  kidneys. 

The  testa  for  the  various  constituents  of  the  urine  must  be 
studied  practically.  (Chemical  Physiology,  p.  22  et  seq.) 

I.  Physical  Characters. 

The  characters  of  the  urine  depend  largely  on  the 
relative  proportion  of  water  and  of  solids  which  are  excreted 
in  it :  at  one  time  it  may  be  very  concentrated,  while  at 
another  time  it  may  be  very  dilute  indeed.  For  this  reason 
its  specific  gravity,  which  depends  upon  the  percentage  of 
solids  in  solution,  varies  within  wide  limits,  being  often  as 
high  as  1030  and  frequently  as  low  as  1005 ;  but  the  average 
specific  gravity  is  about  1020.  It  is  possible  from  the 
specific  gravity  to  form  a  rough  idea  of  the  amount  of  solids 
present,  for  by  multiplying  the  last  two  figures  by  2'22  the 
amount  of  solids  per  1000  parts  is  given. 

Since  the  percentage  of  pigments  in  the  urine  varies  like 
that  of  the  other  constituents,  the  colour  of  the  urine  shows 
wide  divergence  in  the  normal  condition.  A  concentrated 
urine  has  a  dark  amber  colour,  while  a  dilute  urine  may  be 
almost  colourless.  Under  average  conditions  the  urine  has 
a  straw  yellow  colour. 

The  reaction  of  urine  is  normally  acid  in  man,  chiefly 
from  the  presence  of  acid  sodium  phosphate,  NaH2P04,  and 


394 


EXCRETION  OF  MATTER  FROM  THE  BODY     395 

the  degree  of  acidity  varies  with  the  concentration.  But  the 
acidity  of  the  urine  may  also  be  varied  by  different  con- 
ditions. It  is  increased  when  there  is  an  increased  oxidation 
of  proteids,  for  the  sulphuric  acid  and  phosphoric  acid  thus 
formed  have  to  be  neutralised  by  combining  with  the 
alkalies.  It  is  decreased  by  taking  alkalies,  or  by  taking 
such  vegetable  salts  as  the  citrate,  malate  or  tartrate  of 
soda,  because  these  are  oxidised  in  the  body  and  excreted 
as  the  carbonates.  The  NaH2P04  may  then  be  changed 
to  Na2HPO4. 

Urine  is  normally  transparent;  but  when  it  has  stood  for 
a  few  hours,  a  cloud  of  a  mucin-like  substance  is  seen  floating 
in  it.  When  it  is  alkaline  it  is  turbid  from  the  separation 
of  a  white  deposit  of  phosphates  of  lime  and  magnesia.  In 
the  alkaline  urine  of  herbivora  the  white  deposit  is  chiefly 
composed  of  carbonate  of  lime. 

A  brick-red  deposit  of  urates  tends  to  fall  as  the  urine 
cools  when  it  is  concentrated  and  very  acid. 

The    smell    of    urine  is   characteristic,   and    it    may   be 
modified  by  the  ingestion  of  many  different  substances. 
* 

II.  Composition. 

Since  the  relative  amounts  of  water  and  solids  vary 
within  such  wide  limits,  the  percentage  composition  of  urine 
is  of  little  moment.  Under  average  conditions  the  water 
constitutes  about  96  per  cent.,  and  the  solids  about  4  per 
cent.  Of  these  solids,  rather  more  than  half  are  organic, 
rather  less  than  half  are  inorganic.  Since  water  and  solids 
are  derived  from  the  water  and  solids  taken  by  the  indi- 
vidual, the  amounts  excreted  depend  upon  the  amounts 
taken,  and  must  be  considered  in  connection  with  them. 
Thus  if  a  man  takes  little  fluid,  he  will  pass  little  water  in 
the  urine.  If  he  takes  little  food,  a  small  quantity  of  solids 
will  be  excreted  by  the  kidneys.  Since  excretion  and  inges- 
tion must  be  studied  in  relationship  to  one  another,  it  is 
convenient  to  compare  them  during  a  definite  period  of 
time,  and  the  natural  division  into  days  of  twenty-four 
hours  is  generally  adopted. 

Under  ordinary  conditions  the  amount  of  solid  food  taken 


396  HUMAN   PHYSIOLOGY 

per  day  does  not  vary  very  greatly,  but  the  amount  of  fluids 
imbibed  varies  within  much  wider  limits.  For  this  reason, 
while  the  amount  of  water  excreted  in  the  urine  per  diem 
varies  enormously,  the  amount  of  solids  is  more  fixed.  In  a 
man  on  an  average  diet,  it  may  be  stated  that  something 
like  1500  c.cm.  of  water  and  60  to  70  grms.  of  solids  are 
daily  eliminated. 

I.  Nitrogenous  Substances. 

A.  Urea. — The  chemistry  and  mode  of  formation  of  urea 
have  been  discussed  on  p.  369.     Since  it  is  as  urea  that, 
on  an  ordinary  diet,  nearly  90  per  cent,  of  the  waste  nitrogen 
is  eliminated  in  the  urine,  the  amount   excreted   depends 
upon  the  amount  of  proteids  taken  in  the  food.     For  this 
reason,  during  fasting,  the  excretion  of  urea  may  fall  as  low 
as  6  grms.  per  diem,  while  on  a  diet  containing  the  ordinary 
amount  of  proteids,  about  33  grms. — 15*4  grms.  of  nitrogen — 
are  excreted.     On  a  normal  diet  from  86  to  90  per  cent,  of 
the  waste  nitrogen  is  excreted  as  urea,  but,  when  the  nitrogen 
intake  is  decreased,  the  proportion  of  urea-nitrogen  may  fall 
to  as  low  as  60  per  cent. 

When  the  urine  is  allowed  to  stand,  certain  micro- 
organisms are  apt  to  get  into  it,  and  to  cause  a  hydration  of 
the  urea,  whereby  it  is  changed  into  ammonium  carbonate — 

TT  T_T 

0       H         H)  [H 

H   N-O-C-O-N  ' 

H'  IH 

The  urine  is  thus  made  alkaline,  and  the  phosphates  of 
the  earths  are  precipitated.  The  phosphate  of  magnesia 
combines  with  the  ammonia  to  form  ammonio-magnesium- 
phosphate,  NH4MgP04-f  6H2O,  which  crystallises  in  charac- 
teristic prism-like  crystals. 

B.  Non-Urea  Nitrogen. — The  10  or  12  per  cent,  of  nitrogen 
which  on  an  ordinary  diet  is  not  excreted  as   urea  is  dis- 
tributed in : — 

1.  Ammonium  Salts. — About  4  or  5  per  cent,  of  the  total 


EXCRETION  OF  MATTER  FROM  THE  BODY    397 

nitrogen  is  normally  excreted  as  ammonium  salts.  But 
under  certain  conditions  the  proportion  is  greatly  increased. 
Anything  which  causes  an  increased  breaking  down  of 
proteid,  and  an  increased  formation  of  acids,  leads  to  an 
increased  excretion  of  ammonia — the  ammonia  being  formed 
from  the  proteids  to  neutralise  the  acids. 

2.  Diureides. — The  members  of  this  series  of  bodies 
consist  of  two  unmodified  or  modified  urea  molecules,  linked 
together  by  an  acid  nucleus.  The  most  important  of  the 
series  have  as  the  linking  molecule  acrylic  acid,  and  they 
constitute  the  purin  bodies. 


0 

II 
j  H— N— C— N— H  i     Urea. 

C=C — C  =  0        Acrylic  acid  nucleus. 

;  H— N    N— H  ! 

i          \/          I 

C  Urea. 

II 
0 


In  birds  and  reptiles  they  replace  urea  as  the  substances 
in  which  nitrogen  is  chiefly  eliminated.  In  these  animals 
they  are  formed  in  the  liver  from  lactate  of  ammonia, 
derived  from  proteids,  but  in  mammals  they  appear  to  be 
very  largely  derived  from  the  decomposition  of  nucleic  acid. 
Even  when  all  supplies  from  without  of  nucleins  and 
purin  bodies  are  cut  off,  a  certain  amount  of  these  purin 
bodies  is  daily  eliminated.  These  have  been  called  the 
"endogenous"  purins,  while  those  derived  from  the  con- 
stituents of  the  food  are  termed  the  "  exogenous  "  purins.  A 
certain  amount  of  the  purins  formed  are  changed  to  urea 
before  being  excreted,  and,  therefore,  when  disturbances 
of  the  chemical  processes  in  the  liver  occur,  the  purins 
appear  to  be  increased  at  the  expense  of  the  urea. 

Uric  Acid  is  the  most  important  member  of  the  series. 
Its  constitution  is  shown  above. 


398  HUMAN  PHYSIOLOGY 

It  is  an  exceedingly  insoluble  substance  which  tends  to 
crystallise  in  large  irregular  crystals,  and  in  the  urine  these 
are  generally  coloured  brown  by  the  urinary  pigment. 

It  occurs  as  salts  of  sodium  and  potassium,  and,  according  to 
Roberts,  the  acid  salt,  NaHlT,  is  linked  to  a  molecule  of  the 
acid  to  form  NaHU-H2TT,  or,  what  he  calls,  a  quadriurate ; 
but  the  evidence  of  this  is  not  conclusive. 

Although  the  salts  of  uric  acid  are  more  soluble  than  the 
free  acid,  only  a  small  quantity  can  be  dissolved  in  the 
urine.  Apparently  inorganic  salts,  such  as  phosphate  of 
soda,  act  as  solvents.  When  the  urine  cools,  especially  if  it 
is  unusually  acid,  the  urates  tend  to  separate  out  and  fall  as 
a  brick-red  deposit,  which  generally  shows  no  crystalline 
structure  under  the  microscope. 

If  the  urine  has  become  ammoniacal  on  standing,  the 
deposit  frequently  contains  characteristic  spinous  pigmented 
crystals  of  urate  of  ammonia. 

From  their  insolubility,  uric  acid  and  the  urates  tend  to 
form  calculi  or  concretions  in  the  urinary  passages.  The 
presence  of  uric  acid  in  such  concretions  is  recognised  by  the 
murexide  test,  which  depends  upon  the  fact  that  uric  acid 
heated  with  nitric  acid  is  oxidised  to  alloxantin,  which 
strikes  a  purple  colour  with  ammonia,  yielding  murexid — 
the  ammonium  salt  of  purpuric  acid. 

Other  members  of  the  series,  such  as  xanthin  and 
hypoxanthin,  occur  in  the  urine  in  small  quantities. 

Allantoin,  which  occurs  in  the  urine  of  the  foetus,  and  in 
the  urine  of  dogs  after  the  administration  of  nucleic  acid,  is 
a  diureide  in  which  gly  coxy  lie  acid  with  two  carbon  atoms  is 
the  linking  band. 

3.  Greatinin. — Creatinin  is  the  form  in  which  the  creatin 
of  muscle  is  excreted. 

Creatin  is  methyl-guanidin-acetic  acid  (see  p.  43).  By 
dehydration  creatinin  is  produced.  The  amount  excreted  is 
always  small,  and  depends  upon  the  amount  of  muscular 
tissue  broken  down  in  the  body.  According  to  the  in- 
vestigations of  Folin  the  amount  of  creatinin  excreted 
per  diem  on  a  flesh-free  diet  is  very  constant  in  each  in- 
dividual, and  does  not  vary  with  the  amount  of  proteid 
food  taken. 


EXCRETION  OF  MATTER  FROM  THE  BODY    399 
4.  Hippuric  Acid. — This  is  benz-amido-acetic  acid. 


H 


H    0 

_C— C— 0— H 
H 


It  is  formed  from  benzoic  acid  taken  in  the  food  by  link- 
ing it  to  glycocoll — amido-acetic  acid.  This  synthesis  appears 
to  take  place  in  the  kidneys,  for  it  has  been  found  that 
hippuric  acid  is  not  formed  when  these  organs  are  excised, 
and  that,  when  blood  containing  benzoates  is  circulated 
through  them,  hippuric  acid  is  produced.  Its  chief  interest 
is  in  the  fact  that  it  is  one  of  the  first  organic  compounds 
which  were  demonstrated  to  be  formed  synthetically  in  the 
animal  body.  Normally  it  is  present  in  human  urine  in 
very  small  quantities,  but  in  the  urine  of  herbivora  the 
amount  is  considerable,  from  the  presence  of  benzoic  acid 
in  the  fodder.  The  acid  itself  is  insoluble,  and  it  occurs  as 
the  soluble  soda  salts. 


II.  Sulphur-containing  Bodies. 

The  sulphur  excreted  in  the  urine  is  derived  from  the 
sulphur  of  the  proteid  molecule,  and  the  amount  of  sulphur 
excreted  may  be  taken  as  a  measure  of  the  amount  of  pro- 
teid decomposed.  This  is  sometimes  used  as  a  check  upon 
an  estimation  from  the  excretion  of  nitrogen. 

A.  Acid  Sulphur. — The  greater  part  of  the  sulphur  is 
fully  oxidised  to  S03.  (a)  Preformed  Sulphates.  About 
nine-tenths  of  this  is  linked  with  bases  to  form  ordinary 
sulphates. 

(b)  The  other  one-tenth  is  in  organic  combination,  linked 
to  benzene  compounds,  Ethereal  Sulphates.  The  indol, 
skatol,  and  phenol  (see  p.  351),  formed  by  the  putrefaction 
of  proteids  in  the  bowel,  being  excreted  in  the  urine  in  an 
oxidised  form  linked  with  sulphuric  acid.  Indol,  as  already 
shown,  is  related  to  amido-ethyl-benzene. 


40O 


HUMAN   PHYSIOLOGY 


It  is  oxidised  into  indoxyl  thus — 

H 


_N— C— H 

// 
C 

O'H 


This  when  linked  to  sulphate  of  potassium  forms  indoxyl- 
sulphate  of  potassium  or  indican. 


S02OK 


From  skatol,  which  is  methyl-indol,  skatoxyl-sulphate  of 
potassium  is  formed  in  the  same  way. 

These  bodies  are  colourless,  but  when  oxidised  they  yield 
pigments — indican  yielding  indigo  blue,  skatoxyl-sulphate  of 
potassium  yielding  a  rose  colour. 


— 0— H 


— 0— S02OK 


Phenol  is  also  linked  with  sulphate  of  potassium,  and 
excreted  in  the  urine. 

Their  amount  depends  upon  the  activity  of  putrefaction  in 
the  intestine,  and  is  a  good  index  of  its  extent. 


EXCRETION  OF  MATTER  FROM  THE  BODY    401 

When  Dioxybenzene  or  Pyrocatechin  is  formed  in  the  body, 
it  too  is  linked  to  sulphate  of  potas- 
sium and  excreted.  When  urine 
containing  this  substance  stands,  it  be- 
comes oxidised  and  yields  a  greenish 
brown  or  black  pigment. 

B.  Neutral  Sulphur. — A  small  quantity  of  sulphur  is  some- 
times excreted  in  a  less  oxidised  state,  in  the  form  of  neutral 
sulphur.  The  most  important  compound  of  this  kind  is 
cystin,  the  disulphide  of  amido-propionic  acid,  two  molecules 
of  amido-propionic  acid  linked  by  sulphur — 

Amido-propionic  acid 

i 

Sulphur 

I 
Sulphur 

i . 

Amido-propionic  acid 

In  some  individuals  and  in  certain  conditions  of  the  meta- 
bolism not  yet  fully  understood,  the  amount  of  cystin  is 
increased,  and  it  then  tends  to  crystallise  out  in  peculiar 
hexagonal  plates. 

III.  Phosphorus-containing  Bodies. 

The  phosphorus  in  the  urine  is  derived  partly  from  phos- 
phates taken  in  the  food,  and  partly  from  the  nucleins  of 
the  food  and  tissues  and  from  the  bones. 

(a)  Normally  the  phosphorus  is   fully  oxidised  to   P205, 
which  is  linked  to  alkalies  and  earths,  and  excreted  in  the 
urine.     The  most  important  phosphate  is  the  phosphate  of 
soda,   NaH2P04,   which  is   the  chief  factor  in  causing  the 
acidity  of  the  urine.     About  one  quarter  of  the  phosphoric 
acid  is  linked  to  lime  and  magnesia,  and  it  is  these  earthy 
phosphates    which    precipitate    when    the    urine   becomes 
alkaline.      When   the    urine    becomes    ammoniacal,    triple 
phosphate  is  formed  (p.  396). 

(b)  It  is  probable  that  a  small  quantity  of  the  phosphorus 

26 


402  HUMAN   PHYSIOLOGY 

is  excreted  in  organic  compounds,  such  as  glycero-phosphates ; 
but  so  far  these  have  not  been  fully  investigated. 

IY.  Chlorine-containing  Bodies. 

Chloride  of  sodium  is  the  chief  salt  of  the  urine.  It  is 
entirely  derived  from  the  salt  taken  in  the  food,  and  its 
amount  varies  with  the  amount  ingested.  From  10  to  15 
grms.  are  usually  excreted  per  diem  in  a  person  on  normal 
diet. 

In  starvation,  and  still  more  in  fever,  the  tissues  of  the 
body  have  an  extraordinary  power  of  holding  on  to  the 
chlorine,  and  the  chlorides  may  almost  disappear  from  the 
urine. 

Y.  Bases  of  the  Urine. 

Sodium,  potassium,  calcium,  and  magnesium  occur  in  the 
urine  in  amounts  varying  with  the  amounts  taken  in  the 
food.  Generally  speaking  sodium  is  in  excess  of  the  others, 
but  on  a  flesh  diet  and  in  starvation  it  may  fall  below  the 
potassium.  Calcium  and  magnesium  are  present  in  much 
smaller  quantities. 

YI.  Pigments. 

A  brown  hygroscopic  substance,  which  gives  no  bands  in 
the  spectrum,  may  be  extracted  from  urine.  This  has  been 
termed  urochrome.  By  reducing  this,  another  pigment, 
urobilin,  is  produced,  which  gives  definite  bands,  and  which 
is  frequently  present  in  the  urine.  It  is  probably  identical 
with  the  hydrobilirubin  which  has  been  prepared  from  the 
bile  pigments,  and  it  contains  C.  H.  O.  and  N.  The  pigment 
which  gives  the  pink  colour  to  urates  has  been  called  uro- 
erythrin,  and  its  chemical  nature  is  unknown.  Haemato- 
porphyrin  (see  p.  198)  is  normally  present  in  small  traces  in 
the  urine,  but  in  certain  pathological  states  it  is  increased  in 
amount,  and  gives  a  brown  colour  to  the  urine. 

YII.  Nucleo-proteid. 

A  mucin-like  substance  derived  from  the  urinary  passages 
is  always  present  in  small  amounts,  and  forms  a  cloud  when 
the  urine  stands. 


EXCRETION  OF  MATTER  FROM  THE  BODY    403 

YIII.  Carbonic  and  Oxalic  Acids. 

1.  Carbonic  Acid. — Small  amounts  of  this  are  present 
in  urine,  and  after  the  administration  of  citrates,  malates, 
or  tartrates,  the  amount  may  be  considerably  increased, 
and  the  urine  may  then  effervesce  strongly  when  an  acid 
is  added. 

2.  Oxalic  acid  is  a  substance  in  a 
stage  of  oxidation  just  above  that  of 

u 

TT Q Q Q Q jj     carbonic  acid.    It  is  frequently  pre- 
sent in  the  urine  linked  with  lime, 

and  the  lime  salt  tends  to  crystallise  out  in  characteristic 
octohedra,  looking  like  small  square  envelopes  under  the 
microscope.  Under  certain  conditions  these  crystals  assume 
other  shapes.  The  oxalic  acid  of  the  urine  is  chiefly  derived 
from  oxalates  in  vegetable  foods,  but  it  has  been  detected  in 
the  urine  of  animals  on  a  purely  flesh  diet. 

SECRETION  OF  URINE. 

Structure  of  the  Kidney. 

(This  must  be  studied  practically.}  The  kidney  (Fig.  156) 
is  a  compound  tubular  gland,  consisting  of  innumerable 
tubules,  each  made  up  of— 

A  closed  extremity  or  Malpighian  body  (M.B.),  consisting 
of  an  expansion  at  the  end  of  the  tubule — Bowman's  capsule 
—into  which  a  tuft  of  capillary  vessels — the  glomerulus — 
projects.  Extending  away  from  this  is — 

A  proximal  convoluted  tubule  (P.C.T.)  lined  by  pyramidal 
and  granular  epithelial  cells.  This  dives  into  the  medulla 
and  again  ascends  to  the  cortex,  forming  Henle's  Loop 
{H.L.),  which  terminates  in  the  distal  convoluted  tubule. 
This  exactly  resembles  the  proximal  (D.C.T.).  It  opens 
into  a  collecting  tubule  (C.T.)  lined  by  a  low  transparent 
epithelium,  which  conducts  the  urine  to  the  pelvis  of  the 
kidney. 

The  renal  artery  breaks  up  and  gives  off  a  series  of  straight 
branches — the  interlobular  arteries  (IL.A.) — which,  as  they 
run  towards  the  surface,  give  off  short  side  branches  which 
terminate  in  the  glomeruli.  The  efferent  vein  passing  from 


404 


HUMAN   PHYSIOLOGY 


these  breaks  up  again  into  a  series  of  capillaries  between 
the  convoluted  tubules,  and  these  pour  their  blood  into  the 
interlobular  veins  (IL.V.).  This  arrangement  must  help 
to  maintain  a  high  pressure  in  the  capillary  loops  of  the 
glomerular  tuft. 

Physiology  of  Secretion. 

I.  Secretion  in  the  Malpighian  Bodies. — A  consideration 
of  the  structure  of  the  Malpighian  bodies  tends  to  the  conclu- 


FlO.  156. — Diagram  of  the  Structure  of  the  Kidney.  M.P.,  Malpighian 
Pyramid  of  the  Medulla;  M.R.,  Medullary  Ray  extending  into 
Cortex;  L.,  Labyrinth  of  Cortex;  M.B.,  a  Malpighian  Body  con- 
sisting of  the  Glomerular  Tuft  and  Bowman's  Capsule  ;  P.C.T.,  a. 
Proximal  Convoluted  Tubule;  H.L.,  Henle's  Loop  on  the  Tubule; 
D.C.T,,  Distal  Convoluted  Tubule  ;  C.T.,  Collecting  Tubule  ;  R.A., 
Branch  of  Renal  Artery,  giving  off  IL.A..  Interlobular  Artery,  to 
supply  the  Glomeruli  and  the  Convoluted  Tubules ;  IL.  V, ,  Inter- 
lobular Artery  bringing  Blood  back  from  the  Cortex. 

sion  that  in  them  the  water  of  the  blood  must  filter  out  into 
Bowman's  capsule,  under  the  influence  of  the  intravascular 
pressure.  That  they  do  act  in  this  way  is  demonstrated  by 
(a)  the  influence  of  the  blood  pressure  in  the  kidneys  on  the 
flow  of  urine,  and  (6)  the  effect  of  increasing  the  pressure  in 
the  ureters  on  the  flow  of  urine. 


EXCRETION  OF  MATTER  FROM  THE  BODY    405 

(a)  Blood  pressure  in  the  vessels  of  kidney.     The  pressure 
of  blood  in  the  glomerular  tufts  may  be  increased  in  two 
ways — (a)  By  dilating  the  arteries,  and  (b)  by  constricting  or 
occluding  the  veins.     The  former  leads  to  an  increased  pres- 
sure on  the  arterial  side  of  the  capillary  loops  and  to  a  more 
rapid  flow  of  blood,  the  latter  to  a  rise  in  pressure  on  the 
venous  side  and  a  slower  flow  of  blood  (see  p.  269).     In  both 
cases  the  amount  of  blood  in  the  kidney  is  increased,  and  the 
organ  expands ;  but  while  the  former  condition  is  associated 
with  an  increased  flow  of  urine,  the  latter  is  accompanied  by 
a  decreased  or  arrested  flow.     A  high  pressure  in  the  glome- 
ruli,  with  a  rapid  flow  of  blood,  increases  the  flow  of  urine. 
This  may  be  produced  (a)  by  cutting  the  renal  nerves,  which 
maintain  a  tonic  constricting  action  on  the  arterioles ;  (b)  by 
raising  the  general  arterial  pressure  and  thus  forcing  more 
blood  into  the  kidneys. 

The  converse  condition  of  decreased  pressure  in  the  gloine- 
ruli  and  diminished  rate  of  blood-flow  may  be  produced  by 
stimulating  the  renal  nerves  derived  from  the  llth,  12th, 
and  13th  dorsal  nerves,  or  by  causing  a  decrease  of  the 
general  arterial  pressure.  A  fall  of  the  carotid  pressure  to 
about  50  mm.  Hg  in  the  dog  is  generally  sufficient  to  arrest 
the  flow  of  urine.  The  influence  of  such  a  fall  of  pressure  is 
often  seen  in  heart  disease,  where,  as  a  result  of  it,  the  excre- 
tion of  water  by  the  Malpighian  bodies  is  so  impeded  that 
dropsy  supervenes. 

(b)  If  the  ureter  be  ligatured  the  production  of  urine  stops 
when  the  pressure  behind  the  ligature  rises  to  about  50  mm. 
Hg.     This  further  supports  the  view  that  the  formation  of 
urine  in  the  Malpighian  bodies  is  due  to  filtration   under 
pressure. 

The  urine  formed  in  these  bodies  is  alkaline,  as  has  been 
demonstrated  by  applying  an  indicator — a  substance  the 
colour  of  which  is  changed  by  alkalies — to  the  cut  surface 
of  the  kidney ;  and  this  further  supports  the  conclusion  that 
it  is  simply  an  exudation  from  the  blood. 

That  various  substances  in  solution  pass  out  with  the 
water  is  demonstrated  by  the  fact  that  when  haemoglobin  is 
injected  into  the  blood  vessels  it  soon  makes  its  appearance 
in  Bowman's  capsules  and  in  the  urine. 


406  HUMAN   PHYSIOLOGY 

It  may  also  be  demonstrated  by  taking  advantage  of  the 
fact  that  in  the  frog  the  Malpighian  bodies  are  chiefly,  if 
not  entirely,  supplied  from  the  renal  artery,  and  the  tubules 
from  a  portal  vein.  When  sugar  or  commercial  peptone  is 
injected  into  the  circulation,  it  appears  in  the  urine,  but 
if  the  renal  arteries  are  first  ligatured,  it  does  not  appear, 
proving  that  it  is  by  the  gloineruli  that  it  is  passed  out. 

II.  Secretion  in  the  Tubules. — The  alkaline  urine  formed  in 
the  Malpighian  bodies  undergoes  changes  as  it  passes  along 
the  tubules.  It  becomes  acid  and  the  various  solids  must 
be  increased,  for  urine  contains  a  higher  proportion  of  these 
than  does  the  blood.  The  blood  contains  only  about  0'03  per 
cent,  of  urea,  but  the  urine  usually  contains  2  per  cent.  It 
has  been  suggested  that  this  is  due  to  absorption  of  water 
from  the  tubules,  by  which  the  urine  formed  in  the  Mal- 
pighian bodies  become  more  concentrated.  But  there  is  no 
evidence  that  this  takes  place.  On  the  other  hand,  the  fol- 
lowing facts  seem  to  show  that  solids  are  added  to  the  urine 
by  a  process  of  secretion  from  the  cells  of  the  tubules. 

(a)  Uric  acid  crystals  are  frequently  found  in  the  cells  of 
the  convoluted  tubules  of  the  kidney  of  birds.  (6)  Heiden- 
hain,  by  injecting  a  blue  pigment — sulph-indigotate  of  soda 
— into  the  circulation  of  the  rabbit,  demonstrated  that  the 
cells  of  the  convoluted  tubules  take  it  up  and  pass  it  into  the 
urine.  In  the  normal  rabbit  the  whole  of  the  kidney  and  the 
urine  become  blue.  But  if  the  formation  of  urine  in  the 
Malpighian  bodies  is  stopped  by  cutting  the  spinal  cord  in 
the  neck  so  as  to  lower  the  blood  pressure,  then  the  blue  pig- 
ment is  found  in  the  cells  of  the  convoluted  tubules  and  of 
the  ascending  limb  of  Henle's  tubule,  (c)  When  the  Mal- 
pighian bodies  of  the  frog  have  been  thrown  out  of  action  by 
ligaturing  the  renal  arteries,  the  injection  of  urea  still  causes 
a  flow  of  urine  and  the  excretion  of  that  substance  by  the 
tubules. 

This  last  experiment  seems  to  show  that  the  cells  of  the 
convoluted  tubules  are  capable  of  secreting  water,  and  that 
they  can  do  so  is  further  demonstrated  by  the  fact  that,  if 
by  cutting  the  spinal  cord  in  the  neck  the  formation  of 
urine  in  the  Malpighian  bodies  of  a  dog  is  stopped,  the  ad- 
ministration of  caffeine  and  of  some  other  substances  causes 


EXCRETION  OF  MATTER  FROM  THE  BODY    407 

an  increased  flow  of  urine,  although  the  blood  pressure  in  the 
kidney  is  not  raised.  This  is  taken  advantage  of  in  cases  of 
heart  disease,  when  the  secretion  of  urine  is  almost  arrested 
from  low  arterial  pressure,  and  when  dropsy  is  rapidly  ad- 
vancing. Until  the  heart  is  toned  up,  the  kidneys  may  be 
stimulated  to  get  rid  of  water  by  means  of  such  diuretics  as 
caffeine. 

EXCRETION  OF   URINE. 

1.  Passage  from  Kidney  to  Bladder. — The  pressure  under 
which  the  urine  is  secreted  is  sufficient  to  drive  it  along  the 
ureters  to  the  bladder.     If  these  are  constricted  the  pressure 
behind  the  constriction  rises,  and  may  distend  the  ureter  and 
pelvis  of  the  kidney,  but  when  it  reaches  about  50  mm.  Hg 
in  the  dog,  the  secretion  of  urine  is  stopped.     The  muscular 
walls  of  the  ureters  show  a  rhythmic  peristaltic  contraction, 
which  must  also  help  the  onward  passage  of  the  urine  to  the 
bladder. 

2.  Micturition. — As  the  urine  accumulates  in  the  urinary 
bladder  the  rhythmic  contraction  of  the  non-striped  muscle 
becomes  more  and  more  powerful.     These  contractions  are 
chiefly  excited  by  the  fibres  of  the  nervi  erigentes  of  the 
second  and  third  sacral  nerves,  although  fibres  passing  down 
from  the  inferior  mesenteric  ganglion  also  probably  act  either 
in  exciting  or  inhibiting  in  different  animals.     Very  different 
results   as  regards  the   action   of  these   nerves  have   been 
observed,  and  at  present  no  definite  conclusion  as  to  their 
mode  of  action  is  possible.     The  backward  passage  of  the 
urine  into  the  ureters  is  prevented  by  the  oblique  manner 
in  which  these  tubes  pass  through  the  muscular  coat  of  the 
bladder. 

The  passage  of  urine  into  the  urethra  is  at  first  prevented 
either  by  the  oblique  manner  in  which  the  urethra  leaves  the 
bladder,  or  more  probably  by  the  contraction  of  a  strong 
band  of  non-striped  muscle,  the  sphincter  trigonalis.  This 
muscle  or  the  striped  fibres  which  surround  the  membranous 
part  of  the  urethra  are  under  the  control  of  a  centre  in  the 
lumbar  enlargement  of  the  spinal  cord,  and  the  expulsion  of 
urine  must  be  preceded  by  their  relaxation.  In  some  cases 
of  inflammation  of  the  spinal  cord  the  increased  activity  of 


408  HUMAN   PHYSIOLOGY 

the  centre  may  prevent  the  expulsion  of  urine,  while  later  in 
the  disease,  when  the  nerve  structures  have  been  destroyed, 
the  urine  is  not  retained  and  dribbles  away  on  account  of  the 
absence  of  the  tonic  contraction  of  the  muscles. 

The  expulsion  of  the  last  drops  of  urine  is  carried  out  by 
the  rhythmic  contraction  of  the  bulbo-cavernous  muscle, 
while  the  peristaltic  contraction  of  the  bladder  wall  is  assisted 
by  the  various  muscles  which  can  press  upon  the  contents  of 
the  bladder. 

In  man,  in  early  life,  micturition  is  a  purely  reflex  act,  and 
in  the  dog  it  is  perfectly  performed  when  the  spinal  cord  is 
cut  in  the  back.  As  age  advances  the  reflex  mechanism 
comes  to  be  more  under  the  control  of  the  higher  centres, 
and  the  activity  of  the  sphincters  may  be  increased  or 
abolished  as  circumstances  indicate. 


3.  EXCRETION  BY  THE  SKIN. 

The  skin  is  really  a  group  of  organs,  and  some  of  these 
have  been  already  studied.  (The  structure  of  the  skin  and 
its  appendages  must  be  studied  practically.) 

(1)  The  Protective  functions  of  the  horny  layer  of  epi- 
dermis, with  its  development  in  hair  and  nail,  and  of  the 
layer  of  subcutaneous  fat,  are  manifest. 

Hair. — In  man  the  hair  has  largely  ceased  to  have  the  im- 
portant protective  function  it  fulfils  in  many  of  the  lower 
animals,  but  the  muscular  mechanism  by  which  the  position 
of  the  hairs  can  be  modified  still  persists.  Attached  to  each 
hair  follicle  is  a  band  of  non-striped  muscle,  the  arrector  pili, 
which  by  contracting  can  erect  the  hair.  These  muscles  are 
under  the  control  of  the  central  nervous  system,  and  the 
nerve  fibres  have  been  demonstrated  in  the  cat  to  take  much 
the  same  course  as  the  vaso-constrictor  fibres  of  somatic 
nerves  (see  p.  264).  A  hair  after  a  time  ceases  to  grow,  and 
the  lower  part  in  the  follicle  is  absorbed  and  the  hair  is 
readily  detached.  From  the  cells  in  the  upper  part  of  the 
follicle  a  new  down-growth  occurs,  a  papilla  forms,  and  the 
hair  is  regenerated.  In  many  of  the  lower  animals  this 
process  occurs  twice  a  year. 


EXCRETION  OF  MATTER  FROM  THE  BODY    409 

(2)  The  Sensory  functions  have  been  studied  under  the 
Special  Senses  (p.  98  et  seq.). 

(3)  The  Respiratory  action  of  the  skin  in  mammals  is  of 
little  importance. 

(4)  The  Excretory  Function  of  the  Skin. — Three  sets   of 
glands  develop  in  the  skin  —  sweat  glands  and  sebaceous 
glands,  which  are  common  to  both  sexes  and  are  constantly 
active — and  mammary  glands,  which  are  active  in  the  female 
during  the  period  of  suckling. 

A.  Sweat  Secretion — 1.  Sweating. — The  simple  tubular 
sweat  glands  are  exceedingly  numerous.  It  has  been  calcu- 
lated that  a  man  possesses  about  two  and  a  half  million,  and 
that  if  spread  out  they  would  present  a  surface  of  very  great 
extent. 

From  these  glands  a  considerable  amount  of  sweat  is 
poured  out,  but  to  form  any  estimate  of  the  daily  amount 
is  no  easy  matter,  since  it  varies  so  greatly  under  different 
conditions.  Probably  about  1000  c.cm.  is  an  average  amount. 
When  poured  out,  sweat  usually  evaporates,  and  is  then  called 
insensible  perspiration,  but  when  large  quantities  are  formed, 
or  when,  from  coldness  of  the  surface,  or  of  the  air,  or  from 
the  large  quantity  of  watery  vapour  already  in  the  air,  eva- 
poration is  prevented,  it  accumulates,  and  is  called  sensible 
perspiration. 

A  free  secretion  of  sweat  is  usually  accompanied  by  a 
dilatation  of  the  blood  vessels  of  the  skin,  but  this  may  be 
absent,  and  it  may  occur  without  any  sweat  secretion — e.g. 
under  the  influence  of  atropine. 

2.  Nervous  Mechanism  of  Sweat  Secretion — The  sweat 
glands  are  under  the  control  of  the  central  nervous  system. 
This  may  be  very  conveniently  studied  in  the  cat,  in  which 
animal  the  sweat  glands  are  chiefly  in  the  pads  of  the  feet. 
If  a  cat  be  put  in  a  hot  chamber  it  sweats  on  the  pads  of  all 
its  feet.  But  if  one  sciatic  nerve  be  cut  the  foot  supplied 
remains  dry.  If  the  cat  be  placed  in  a  warm  place  and  the 
lower  end  of  the  cut  sciatic  stimulated,  a  secretion  of  sweat 
is  produced.  These  sweat-secreting  fibres  all  pass  through 
the  sympathetic  ganglia,  and  back  into  the  spinal  nerves. 


4io  HUMAN  PHYSIOLOGY 

Those  to  the  leg  and  foot  come  from  the  upper  lumbar  region, 
those  for  the  hand  and  arm  from  the  fifth  to  the  eighth  cervical 
nerve,  and  those  for  the  head  chiefly  from  the  medulla. 

The  centres  presiding  over  these  nerves  have  not  been 
definitely  located.  But  they  are  capable  of  (a)  reflex  stimula- 
tion, as  when  pepper  is  taken  into  the  mouth ;  and  (6)  of 
direct  stimulation  by  a  venous  condition  of  the  blood,  as  in 
the  impaired  oxygenation  of  the  blood  which  so  frequently 
precedes  death  as  the  respirations  fail. 

But  even  after  the  nerves  to  the  sweat  glands  are  cut,  the 
glands  can  be  stimulated  by  certain  drugs — e.g.  pilocarpin. 
The  action  of  heat  seems  also  to  be  chiefly  peripheral,  setting 
up  an  unstable  condition  of  the  gland  cells  so  that  they  re- 
spond more  readily  to  stimulation. 

3.  Chemistry  of  Sweat. — Sweat  is  a  clear,  watery  fluid, 
which,  when  pure,  has  a  neutral  or  faintly  alkaline  reaction, 
but  which  is  generally  acid  from  admixture  with  the  sebaceous 
secretion.  Its  specific  gravity  is  low,  about  1004,  and  it 
contains  less  than  2  per  cent,  of  solids,  of  which  the  chief  is 
NaCl.  Of  the  organic  solids,  urea  is  the  most  important. 
About  4  or  5  per  cent,  of  the  total  nitrogen  excreted  from 
the  body  is  thrown  off  by  the  skin  in  this  form,  and,  when 
the  action  of  the  kidneys  is  interfered  with,  very  considerable 
quantities  may  be  eliminated.  The  sweat  nearly  always  con- 
tains epithelial  squames  from  the  epidermis,  and  oil  globules 
derived  from  the  sebum. 

B.  Sebaceous  Secretion. — The  sebaceous  glands  are  simple 
racemose  glands  which  open  into  the  hair  follicles,  and  their 
function  is  to  supply  an  oily  material  to  lubricate  the  hairs. 
This  secretion  is  produced  by  the  shedding  and  breaking 
down   of  the   cells  formed   in  the  follicles  of  the  glands. 
Those  lining  the  basement  membrane  are  composed  of  proto- 
plasm and  actively  divide,  but  the  cells  thrown  off  into  the 
lumen  of  the  follicle  disintegrate  and  become  converted  into 
a  semi-solid  oily  mass,  which  consists  of  free  fatty  acids  and 
of  neutral  fats  of  glycerine  and  of  cholesterin.     These  latter 
are  the  lanolins,  which  differ  from  ordinary  fats  in  being 
partly  soluble  in  water.     Free  cholesterin  is  also  present  in 
the  sebum. 

C.  Milk    Secretion  —  1.  Physiology. — JBefore   pregnancy 


EXCRETION  OF  MATTER  FROM  THE  BODY  411 

occurs  the  mammary  glands  are  largely  composed  of  fibrous 
tissue,  with  a  large  amount  of  fat,  in  which  run  the  branch- 
ing tubules  of  the  glands  as  small  solid  blocks  of  cells 
completely  filling  the  lumen. 

As  pregnancy  advances  these  tubules  grow  outwards  and 
increase,  and  the  cells  begin  to  divide,  some  remaining 
attached  to  the  basement  membrane,  some  coming  to  lie 
in  the  middle  of  the  tubules.  These  latter  undergo  a  fatty 
change  and  break  down,  and  they  are  shed  in  the  first  milk 
which  is  secreted,  which  is  known  as  the  colostrum.  The 
cells  left  upon  the  basement  membrane  elaborate  the  con- 
stituent of  milk,  and  the  presence  of  fat  globules  in  their 
protoplasm  is  very  manifest. 

The  milk,  after  being  secreted,  collects  in  the  ducts  of  the 
glands  and  in  the  sinus  below  the  nipple,  and  is  expelled 
from  these  by  the  contraction  of  the  muscular  fibres  in  their 
walls,  and  by  the  suction  of  the  young  animal.  The  excre- 
tion of  milk  from  the  ducts  is  directly  under  the  control  of 
the  nervous  system,  but  the  evidence  as  to  the  way  in  which 
the  central  nervous  system  influences  the  secretion  of  milk 
is  by  no  means  satisfactory.  Clinical  experience  shows  that 
it  is  profoundly  modified  by  nervous  changes,  but  so  far 
stimulation  of  the  nerves  to  the  glands  has  not  yielded 
definite  results. 

2.  Characters  of  Milk. — The  white  colour  of  milk  is  due 
to  its  being  a  fine  emulsion  of  fat  globules  floating  in  a  clear 
plasma.  Hence,  when  the  cream  is  removed,  milk  becomes 
less  white  and  less  opaque.  Its  specific  gravity  is  about 
1030,  and  in  man  and  herbivorous  animals  its  reaction  is 
alkaline. 

Its  composition  in  the  human  subject  and  in  the  cow  is 
given  on  p.  318. 

Proteids. — The  chief  proteid  is  caseinogen,  which  exists  as 
a  soluble  calcic  compound.  It  is  of  the  nature  of  a  phospho- 
proteid,  but  contains  very  little  phosphorus,  and  does  not 
yield  purin  bases.  It  is  not  coagulated  on  boiling,  but  its 
combination  with  lime  is  split  under  the  action  of  acids  and 
the  casein  is  precipitated.  Under  the  influence  of  rennet  it 
splits  into  whey  albumin  and  paracasein  lime,  which  is  in- 
soluble, and  separates  out  as  a  curd  which  generally  holds 


4i2  HUMAN   PHYSIOLOGY 

the  fat  of  the  milk,  and  is  therefore  white.  From  the  curd 
a  colourless  whey  containing  albumin  and  milk  sugar  exudes. 

Fats. — Olein  is  the  chief  fat,  but  fats  of  the  lower  fatty 
acids  are  also  present.  They  exist  in  a  fine  state  of  sub- 
division, suspended  in  the  milk  plasma,  and  each  globule  is 
apparently  surrounded  by  a  thin  covering  of  proteid,  which 
has  to  be  removed  by  the  action  of  an  acid  or  an  alkali  before 
the  fat  can  be  extracted  with  ether. 

Sugar. — The  disaccharid  lactose  is  the  sugar  of  milk 
(see  p.  317).  Under  the  action  of  various  micro-organisms 
it  is  split  up  to  form  lactic  acid,  thus  causing  the  souring  of 
milk. 

Phosphorus  Compounds. — In  addition  to  caseinogen,  milk 
contains  other  organic  phosphorous  compounds.  Among 
these  is  lecithin  and  a  compound  which  has  been  called 
phosphocarnic  acid,  the  constitution  of  which  is  not  fully 
understood.  In  human  milk  the  greater  part  of  the 
phosphorus  is  in  organic  combinations,  while  in  cow's  milk 
the  amount  in  inorganic  compounds  is  much  greater. 

Milk  is  specially  rich  in  calcium  and  potassium,  but  the 
amount  of  iron  in  milk  is  very  small,  and  therefore,  when 
the  child  has  used  up  the  store  of  iron  which  it  has  in  its 
body  at  birth,  it  is  necessary  to  replace  the  milk-diet  l>y 
foods  containing  more  iron. 


PART    III 

SECTION  XII 
REPRODUCTION 

So  far  the  animal  has  been  studied  simply  as  an  individual. 
But  it  has  also  to  be  regarded  as  part  of  a  species,  as  an  entity 
which  has  not  only  to  lead  its  own  life,  but  to  transmit  that 
life  to  offspring. 

The  various  problems  of  reproduction  have  been  already 
studied  by  the  student  in  connection  with  biology,  and  it  is 
here  sufficient  to  indicate  some  of  the  main  points  in  the 
physiology  of  the  process  in  mammals. 

(The  structure  of  the  organs  of  reproduction  must  be 
studied  practically.) 

While  the  individual  is  actively  growing,  the  reproductive 
organs  are  quiescent ;  but  when  puberty  is  reached,  they  begin 
to  perform  their  functions — the  testes  to  produce  sperma- 
tozoa, the  ovary  to  produce  mature  ova. 

The  removal  of  the  sexual  organs  in  the  young  animal 
leads  to  arrest  in  the  development  of  the  special  sexual 
characters,  especially  in  the  male,  in  which  these  characters 
are  generally  best  marked.  Simple  ligature  of  the  vas 
deferens  has  not  this  effect. 

The  genital  gland  in  both  sexes  is  formed  from  a  longi- 
tudinal thickening  or  ridge  at  the  posterior  part  of  the 
coelom  or  peritoneal  cavity.  Over  this  ridge  the  endothelium 
thickens  and  becomes  epithelial-like  in  structure.  Groups 
of  cells  grow  down  into  the  tissue  below. 

In  the  ovary  one  of  these  cells  in  a  group  takes  a  central 
position  and  forms  the  ovum,  while  the  other  cells  get 
arranged  around  it  to  form  the  zona  granulosa,  the  whole 

group  constituting  a  Graaiian  follicle. 

413 


4i4  HUMAN   PHYSIOLOGY 

In  the  testis  the  groups  of  cells  form  seminiferous  tubules, 
in  which  the  spermatozoa  or  male  elements  are  developed. 

Ovary. — In  the  adult  the  ovaries  are  oval  structures  covered 
by  a  columnar  germinal  epithelium.  In  the  stroma  are 
seen  Graafian  follicles  in  different  stages  of  development. 
The  central  cell,  the  ovum,  enlarges.  The  nucleus  becomes 
prominent,  and  is  sometimes  called  the  germinal  vesicle. 
The  nucleolus  is  also  large,  forming  the  germinal  spot.  The 
protoplasm  becomes  encased  in  a  transparent  capsule — the 
zona  pellucida.  The  cells  of  the  zona  granulosa  multiply, 
and  fluid  (the  liquor  folliculi)  appears  among  them, 
dividing  them  into  a  set  attached  to  the  capsule  of  the 
follicle  and  a  set  surrounding  the  ovum.  When  the  follicle 
is  ripe  it  projects  on  the  surface  of  the  ovary,  and  finally 
bursts,  setting  free  the  ovum  into  the  peritoneal  cavity.  It 
passes  into  the  trumpet-shaped  fimbriated  upper  end  of  the 
Fallopian  tube,  through  which  it  reaches  the  uterus. 

Testis. — In  the  adult  this  is  enclosed  in  a  dense  fibrous 
capsule  —  the  tunica  albuginea.  Posteriorly  this  is 
thickened,  and  forms  the  corpus  Highmori.  From  it  pro- 
cesses extend  and  form  a  supporting  framework.  In  the 
spaces  are  situated  the  seminiferous  tubules,  which  open 
into  irregular  spaces  in  the  corpus  Highmori — the  rete  testis, 
from  which  the  efferent  ducts  (vasa  efferentia)  pass  away  to 
join  together  to  form  the  vas  deferens. 

In  the  seminiferous  tubules  the  spermatozoa  are  produced. 
Some  of  the  lining  cells  divide  into  two,  forming  a  sup- 
porting cell  and  a  spermatogen.  The  latter  divides  and 
subdivides  till  a  group  of  cells  lie  on  the  top  of  the  sup- 
porting cell.  These  are  the  spermatoblasts.  In  each  sper- 
matoblast  the  nucleus  elongates  and  passes  to  the  attached 
extremity,  the  protoplasm  decreases  in  amount,  and  a  long 
cilium  develops  from  the  free  end,  and  the  spermatozoon 
is  thus  produced. 

Semen. — When  the  testes  have  become  active,  the  glands 
of  the  prostate  increase  and  produce  a  fluid  which,  with  the 
spermatozoa,  forms  the  semen. 

Menstruation. — In  the  case  of  the  female  the  maturation 
of  the  ova  is  accompanied  by  changes  in  the  uterus.  Every 
four  weeks  the  superficial  layers  of  the  mucous  membrane  of 


REPRODUCTION  415 

the  uterus  break  down,  and  are  shed  during  the  menstrual 
period.  After  the  menstrual  period,  the  membrane  again 
regenerates. 

Impregnation  is  effected  by  the  transmission  of  sperma- 
tozoa into  the  passages  of  the  female.  For  this  purpose 
erection  of  the  penis  is  brought  about  reflexly  through  a 
centre  in  the  lumbar  enlargement  of  the  cord,  the  outgoing 
nerves  being  the  nervi  erigentes,  which  dilate  the  arterioles, 
and  the  internal  pudics  supplying  the  transversus  perinei 
and  bulbo-cavernous  muscles  by  which  the  veins  are 
constricted. 

The  semen  is  ejected  by  a  rhythmic  contraction  of  the 
bulbo-cavernous  and  other  perineal  muscles,  an  action  which 
is  also  presided  over  by  a  centre  in  the  lumbar  region  of  the 
cord  (p.  150). 

The  spermatozoon  meets  the  ovum  in  the  Fallopian  tube, 
or  upper  part  of  the  uterus. 


DEVELOPMENT. 
1.   Early  Stage. 

It  is  unnecessary  here  to  describe  the  changes  in  the 
ovum  before  or  immediately  after  its  conjugation  with 
the  spermatozoon,  since 
they  are  so  fully  dealt 
with  in  all  works  on 


The  mammalian  ovum 
is  holoblastic,  that  is 
undergoes  complete  seg- 
mentation, and  forms  a 

mulberry -like    mass    of  JSK^g™  B. 

cells  (Fig.  157,  A).    The 

Cells    then    get     disposed    FIG.  157.—^1.  Ovum  with  central  cells  forming 

in    tWO     SetS,    a    layer    of  Blastoderm.     B.  Blastoderm  now  made  up 

,.  •,,  of    two    layers    of    cells  —  Epiblast    and 

small   surrounding   cells         Hypobiast. 

and  a  set  of  large  central 

cells.      These   latter  spread  out  at  one  pole  to  form   the 


4i6 


HUMAN   PHYSIOLOGY 


blastoderm,  and  dispose  themselves  in  three  layers — the 
epiblast,  mesoblast,  and  hypoblast  (Fig.  158).  From  these 
layers  the  various  parts  of  the  body  are  derived  as  follows : — 
I.  Epiblast. — Nervous  system,  epidermis,  and  appendages. 
Epithelium  of  mouth,  nose,  naso-pharynx,  and  all  cavities 

and  glands  opening  into  them, 
and  the  enamel  of  teeth. 

II.   Hypoblast.  —  Epithelia 
of  (a)  alimentary  canal  from 
back  of  mouth  to  anus  and  of 
all  its  glands ;  (b)  of  Eustachian 
tube  and   tympanum ;  (c)   of 
trachea    and    lungs  ;    (d)    of 
thyroid    and     thy m us ;     and 
(e)  of  urinary  bladder  and  urethra. 
III.  Mesoblast. — All  other  structures. 

By  the  formation  of  a  vertical  groove  down  the  back  of 
the  blastoderm,  a  tube  of  epiblast  cells  (the  neural  canal)  is 
enclosed,  from  which  the  nervous  system  develops  by  the 


FIG.  158. — Transverse  section  of  more 
advanced  Blastoderm,  to  show  Epi- 
blast, Mesoblast,  and  Hypoblast, 
formation  of  Neural  Groove  and 
splitting  of  the  Mesoblast. 


FlQ.  159. — Longitudinal  Section  through  Embryo  to  it  sinking  down  into  ovum  and 
the  formation  of  the  Amnion,  am.  In  the  Mesoblast  round,  all.,  the  Allantois, 
the  blood  vessels  grow  out  to  form  the  placenta. 

conversion  of  some  of  the  cells  into  neurons,  and  others 
into  neuroglia  cells  (Fig.  158). 

The  mesoblast  on  each  side  of  this  splits,  and  the  outer  part, 
with  the  epiblast,  goes  to  form  the  body  wall  (Somatopleur), 
while  the  inner  part  with  the  hypoblast  gets  tucked  in  to  pro- 
duce the  alimentary  canal  (Splanchnopleur)  (Fig.  158). 

The  developing  embryo  sinks  into  the  ovum,  and,  as  a 
result  of  this,  the  somatopleur  folds  over  it  and,  uniting 
above,  encloses  it  in  a  sac — the  amniotic  sac  (Fig.  159,  am.), 
which  becomes  distended  with  fluid — the  amniotic  fluid,  in 
which  the  embryo  floats  during  the  later  stages  of  its  develop- 


REPRODUCTION 


ment,  and  which  acts  as  a  most  efficient  protection  against 
external  violence. 

2.  Attachment  to  the  Mother. 

The  ovuin  gets  enclosed  in  the  uterine  mucous  membrane, 
which  regenerates  round  it  as  the  decidua  reflexa  after  men- 
struation (Fig.  160,  D.K). 

Almost  as  soon  as  the  ovum  is  embedded  in  the  maternal 
mucous  membrane,  it 
becomes  surrounded 
by  a  nucleated  mass 
of  protoplasm  —  the 
trophoblast,  formed  of 
the  cells  of  the  ecto- 
derm, and  this  probably 
transfers  nourishment 
from  the  mother  to 
the  ovum.  At  the 
end  of  about  a  fort- 
night, the  mesoblast 
of  the  embryo  extends 
out  in  a  number  of 
finger  -  like  processes 
into  the  trophoblast, 
and  soon  afterwards 
blood  vessels  shoot 
into  these,  and  the 
chorionic  villi  are 
formed  (Fig.  161). 
The  precise  origin  of  the  first  blood  vessels  in  these  is 
not  known,  but  ultimately  they  are  derived  from  the 
allantoic  arteries  which  pass  out  from  near .  the  posterior 
end  of  the  hind  gut.  As  the  villi  grow,  the  blood  vessels 
in  the  maternal  mucosa  are  dilating,  and  the  capillaries 
form  large  sinuses  or  blood  spaces.  Into  these  the 
chorionic  villi  pass,  and  thus  the  loops  of  foetal  vessels 
hang  free  in  the  maternal  blood,  and  an  exchange  of  material 
is  possible  between  the  mother  and  foetus.  The  placenta 
thus  formed  acts  as  the  fcetal  lung,  giving  the  embryo  the 
necessary  oxygen  and  getting  rid  of  the  waste  carbon  dioxide. 

27 


FIG.  160. — Longitudinal  Section  through  the 
human  uterus  and  ovum  at  the  fifth  week  of 
pregnancy.  D.S.,  Decidua  serotina,  which 
will  become  the  placenta;  D.R.,  Decidua 
reflexa ;  D.  V. ,  The  uterine  mucous  membrane 
called  the  Decidua  vera. 


4i8 


HUMAN   PHYSIOLOGY 


JV.S.P 


FlQ.  161.  —  Longitudinal  Section 
through  the  tip  of  a  villus  of 
the  human  placenta,  covered  by 
its  trophoblast  layer,  and  con- 
taining a  loop  of  blood  vessels, 
and  projecting  into  a  large  blood 
sinus,  I.V.S.,  in  the  maternal 
mucosa. 


It  is  the  foetal  alimentary  canal  supplying  the  necessary 
material  for  growth  and  development;  and  it  is  the  foetal 
kidney  through  which  the  waste  nitrogenous  constituents  are 

thrown  otf.  In  the  mesoblast, 
through  which  the  allantoic 
arteries  pass  out,  a  large  vesicle 
filled  with  fluid,  and  at  first 
communicating  with  the  pos- 
terior gut,  is  developed.  This 
is  the  allantois  (Fig.  159,  ////.). 


3.  Foetal  Circulation. 

Perf°rmance  °f  these 
functions  by  the  placenta  is 
associated  with  a  course  of 
circulation  of  the  blood  some- 
what different  to  that  in  the 
post-natal  state  (Fig.  162). 

The  blood  coming  from  the 
placenta  to  the  foetus  is  collected 
into  a  single  umbilical  vein  which  passes  to  the  liver.  This 
divides  into  the  ductus  venosus,  passing  straight  through 
the  organ,  and  into  a  series  of  capillaries  among  the  cells. 
From  these  the  blood  flows  away  in  the  hepatic  vein  to  the 
inferior  vena  cava,  which  carries  it  to  the  right  auricle.  In 
this  it  is  directed  by  a  fold  of  endocardium,  through  the 
foramen  ovale,  a  hole  in  the  septum  between  the  auricles, 
and  it  thus  passes  to  the  left  auricle,  and  thence  to  the  left 
ventricle,  which  drives  it  into  the  aorta,  and  chiefly  up  to 
the  head.  From  the  head  the  blood  returns  to  the  superior 
vena  cava,  and,  passing  through  the  right  auricle,  enters  the 
right  ventricle,  which  drives  it  into  the  pulmonary  artery. 
Before  birth  this  artery  opens  into  the  aorta  by  the  ductus 
arteriosus,  while  the  branches  to  the  lungs  are  still  very 
small  and  unexpanded.  In  the  aorta,  this  impure  blood 
from  the  head  mixes  with  the  purer  blood  from  the  left  ven- 
tricle, and  the  mixture  is  sent  to  the  lower  part  of  the  body 
through  the  descending  aorta.  From  each  iliac  artery  an 
umbilical  artery  passes  off,  and  these  two  vessels  carry  the 
blood  to  the  placenta. 


REPRODUCTION 


419 


When  the  child  is  born,  the  flow  of  blood  between  it  and 
the  mother  is  arrested,  the  umbilical  cord  being  tied.  As  a 
result  of  this  the  respiratory  centre  is  no  longer  supplied 
with  pure  blood,  and  is 
stimulated  to  action.  The 
lungs  expand  and  the 
blood  flows  through  them. 

In  the  ductus  venosus  a        a  -KC- 

clot  forms  and  the  vessel 
becomes  obliterated.  The 
ductus  arteriosus  also 
closes  up,  and  the  fora- 
men ovale  is  occluded. 
The  circulation  now  takes 
the  normal  course  in 
post-natal  life. 

Our  knowledge  of  the 
differences  between  the 
physiological  processes 
in  embryonic  and  in 
extra- uterine  life  is  still 
very  imperfect,  and  the 
subject  cannot  be  further 
discussed  here. 

$.  Gestation  and 
Delivery. 

The  child  remains  in 
the  uterus  for  nine 
months,  and  at  the  end 
of  that  period  it  is 
expelled  during  labour. 
Labour  may  be 


FIG.  162.— Scheme  of  Circulation  in  the 
Foetus,  u.v.,  umbilical  vein  ;  d.v.,  ductus 
venosus  ;  p.v.c.,  inferior  vena  cava  pour- 
ing blood  through  the  right  auricle  and 
through  the  foramen  ovale,  jjf.o. ,  into  the 
left  heart;  a.v.c.,  superior  venfa  cava  bring- 
ing blood  from  head  to  pass  through  the 
right  side  of  the  heart,  and  through  the 
ductus  arteriosus,  d.a. 


divided 
into  three  stages.     In  the 

first  stage  the  uterus  passes  into  contractions  at  intervals, 
and  the  lower  part  or  cervix  is  dilated.  In  the  second  stage 
the  contractions  become  stronger,  and  with  the  help  of  the 
contractions  of  the  abdominal  muscles,  the  child  is  expelled 
through  the  vagina.  After  this  the  uterus  is  usually 


420  HUMAN   PHYSIOLOGY 

quiescent  for  a  short  time,  and  then  contractions  supervene, 
and  the  placenta  and  lining  of  the  uterus  are  expelled  as 
the  after-birth.  These  uterine  contractions  are  presided  over 
by  a  nerve-centre  in  the  lumbar  enlargement  of  the  cord,  and 
in  all  probability  the  nervi  erigentes  play  an  important  part 
in  their  production. 


APPENDIX 

SOME  ELEMENTARY  FACTS  OF  ORGANIC  CHEMISTRY 


THE  following  elementary  facts  may  help  the  student  who  has 
neglected  the  study  of  the  outlines  of  Organic  Chemistry. 

Organic  compounds  are  built  round  the  four-handed  carbon  atom 

— C— 

I 

When  each  hand  links  to  the  one-handed  hydrogen  atom, 
METHANE —  H 


H— C— H 


is  formed. 

H 

By  taking    away    a  hydrogen   atom  from  two    Methane    mole- 
cules and  linking  these  molecules  together 
ETHANE —  H    H 

i    i 

H — C — C — H         is  produced. 

H     H 

By  further  linking  more  and  more  of  these  molecules  together, 
similar  molecules  containing  three,  four,  five  or  more  carbon  atoms 
are  produced. 

When  each  carbon  has  its  due  proportion  of  hydrogen  atoms  it 
is  saturated,  but  if  two  hydrogen  atoms  are  let  go,  the  unoccupied 
hands  of  the  carbon  may  join  and  form  an  unsaturated  molecule, 
thus : — 

Ethane  becomes  Ethylene         H    H 

H— C=C— H 

When  one  hydrogen  atom  is  taken  away  and  the  molecule  has  a 
hand  ready  to  link  with  some  other  substance  a  radical  is  consti- 
tuted, and  these  are  known  as  METHYL,  ETHYL,  &c. 


422  APPENDIX 

Alcohols.  —  When  the  two-handed  oxygen  atom,  —  O  —  linked  to 
hydrogen  H  —  and  thus  forming  the  hydroxyl  molecule  —  OH  is 
linked  to  the  vacant  hand  of  the  radical,  an  alcohol  is  formed,  e.g.  — 


—  C 


H—  C—  C—  O—  (H)         Ethyl  Alcohol. 


When  the  terminal  carbon  is  thus  oxidised  a  Primary  Alcohol 
is  formed — but  if  a  middle  carbon  atom  is  oxidised,  a  Secondary 
Alcohol  is  produced— 
H  OH  H 

I       I       I 
H— C — C — C— H         Secondary  Propyl  Alcohol. 

Aii 

But  the  oxidation  may  involve  more  than  one  carbon  atom  and 
thus  the  atomic  Polyvalent  Alcohols  are  produced— 
OH     OH   OH 

I         I         I 
H  —  C  —  C  —  C  —  H          Glycerin. 

I          I          I 
H       H       H 

Aldehydes. — When  from  a  Primary  Alcohol  the  two  hydrogens 
in  brackets  are  removed,  the  vacant  hand  of  the  oxygen  links  to 
the  vacant  hand  of  the  carbon  to  form  an  Aldehyde — 
H 

H— C— C=0         Ethyl  Aldehyde. 

H  (H) 

Ketones. — These  are  formed  in  the  same  way  from  the  Secondary 
Alcohols,  thus : — 
H    0    H 

r<     Ji     n     XT  Acetone,  the  Ketone  of  Secondary 

I  I  Pr°Pyl  AlcohoL 

H          H 

Acids. — If  the  hydrogen  of  the  Aldehyde  in  brackets  is  replaced 
by  hydroxyl  — OH  an  acid  is  produced— 

H    :    0 

i  i  n 

H— C-;-C— 0— H         Acetic  acid 

I   i 
H  : 


APPENDIX  423 

The  carboxyl  group  to  the  right  of  the  dotted  line  is  characteristic 
of  the  acids. 

The  oxidation  may  be  carried  on  at  each  end  of  the  line  and  the 
divalent  acids  are  thus  produced 

O    0 

II      II 
H—  0—  C—  C—  0—  H         Oxalic  acid. 

If  in  the  radical  of  one  of  these  acids  a  hydrogen  is  replaced  by 
hydroxyl  —  OH  an  oxi-acid  is  formed,  thus  :  — 

H    H    0 

I       I       II 
H—  C—  C—  C—  0—  H         Propionic  acid 


may  be  converted  to  the  two  Lactic  acids  called  (a)  and  (b)  oxy- 
propionic  acid,  according  to  the  carbon  which  is  oxidised 

H  OH  O 

I       I      I 
H—  C—  C—  C—  O—  H 

I       I 
H    H 

and 

OH   H    0 

I  II 

H—  C—  C—  C—  0—  H 

H    H 

Similar  oxy-acids  are  formed  from  the  divalent  acids. 

BENZENE  COMPOUNDS. 

An  important  series  of  carbon  compounds  contain  a  ring  of  six 
carbons,  each  with  an  unsatisfied  affinity,  thus  :  — 


_/  V 
J    L 

V 


When  each  hand  holds  a  hydrogen,  benzene  is  formedl 


424  APPENDIX 

These  hydrogens  may  be  replaced  by  various  molecules  giving 
rise  to  a  large  series  of  different  compounds. 

NITROGEN-CONTAINING  COMPOUNDS. 

Ammonia. — The  three-handed  Nitrogen  by  linking  with  three 
hydrogens  forms  Ammonia, 

H 

H— N— H 

If  one  of  these  hydrogens  is  removed,  Amidogen,  which  can  link 
with  other  molecules,  is  produced. 

Amido  Acids. — If  one  of  the  hydrogen  atoms  in  the  radical  of 
an  acid  is  replaced  by  amidogen  a  mon-amido  acid  is  formed,  thus  : — 

H     0 
Hx         |      || 

\N_C-C— 0—H         Amido  acetic  acid. 
H/          | 
H 

When  two  hydrogen  atoms  are  thus  replaced,  a  di-amido  acvl  is 
produced — 

NH0  NH,    0 

i     r  ii 

H — C  -  -  C  — C — 0 — H         Di-amido  propionic  acid. 

H        H 

Amides. — If    the   amidogen   molecule   takes   the   place   of   the 
hydroxyl  in  the  carboxyl  of  an  acid  an  amide  results,  thus  : — 
0 

II 
H— C— 0— H          Formic  acid. 

0 


/ 
H— C— N< 

\H 


H 

Formamide. 


From  the  dibasic  carbonic  acid, 

0 

H— O— C— 0— H 
is  formed 

0 

Hv  ||         /H 

\N— C— N< 

H/  XH 

the  important  substance  urea. 


APPENDIX  425 

Urea  molecules  may  link  together — 

(a)  By  dropping  hydrogens  when  Biuret  is  produced, 

0    H     H     0 

Hv  ||  |       ||  H 

\N— C— N— N— C— N< 

is/  XH 

(b)  By  holding  on  to  an  intermediate  radical  of  an  acid,   e.g. 
the  unsaturated  three  carbon  acrylic  acid.     These  are  Diureides  of 
which  the  most  important  is  Uric  Acid. 

0 

H— N— C— N— H; 

0=0— C=0 

I       I 
H— N   N— H 

Y 
4 


INDEX 


INDEX 


ABDOMINAL  breathing,  283 
Abducens  nerve,  159 
Absorption  of  carbohydrates,  359 
channels  of,  358 
of  fats,  359 
of  food,  357,  381 
of  food  proteids,  358 
from  the  stomach,  340 
Accessorius,  158 
Accommodation,  positive,  111  et  seq. 

range  of,  114 
Acetone,  422 
Achromatin,  18 
Achroo-dextrin,  329 
Acid  proteate,  334 

sulphur  in  the  urine,  399 
Acids,  divalent,  423 

in  gastric  juice  (see  Gastric  juice) 
organic,  422 
Acinus,  25 

Acrylic  acid,  397,  425 
Action,  reflex,  87,  88,  91 
latent  time  of,  88 
voluntary,  88,  92 
Addison's  disease,  384 
Adipose  tissue,  31 
Adrenalin,  386 
JEthalium  septicum,  16 
After-birth,  420 
Air  (see  Complemental,  &c.) 
Air  vesicles  in  the  lungs,  277 
Albumin,  11 

Albuminoids,  energy  value  of,  315 
Albumose,  11 

Alcohol  as  a  food  accessory,  382 
Alcohols,  chemistry  of,  422 
polyvalent,  422 
primary,  422 
secondary,  422 
Aldehydes,  422 
Alimentary  canal,  324  et  seq. 
bacterial  action  in,  350 
development  of,  416 
nerve  supply  of,  327 
physiology  of,  328 
structure,  324 
Allantoic  arteries,  417 
Allantoin,  398 
Allantois,  418 
Alloxantin,  398 


Amides,  424 
Amido-benzene,  351 
Amido-ethane-sulphuric  acid  (tauro- 

cholic  acid),  342 
Amido-isethionic  acid,  352 
Amido-propionic  acid,  401 
Amitotic  division,  21 
Ammonia,  11,  424 

Ammonio-magnesium  phosphate,  396 
Ammonium  salts  in  urine,  396 
Amniotic  fluid,  416 

sac,  416 

Amoaboid  movement,  193 
Amount  of  respired  air,  286 
Ampulla,  165 
Amyl  nitrite  and  influence  on  blood 

pressure,  260 

Amylolytic  period  of  gastric  diges- 
tion, 334 
Amylopsin,  347 
Anabolic  phenomena,  8 
Anacrotic  pulse,  252 
Analysis  of  alveolar  air,  302 
Anelectrotonus,  48 
Annulus  of  Vieussens,  243 
Anterior  corpora  quadrigemina,  156, 

167 

pyramids  of  the  medulla,  156 
Antero-lateral  ascending  tract,  153, 

156,  162 

descending  tract,  153,  157 
Antitoxin,  392 
Anus,  326 
Apex  beat,  223 
Appendix,  auricular,  left,  218 
Appendix,  vermiform,  326 
Apocodeine,  143,  263,  385 
Aqueous  humour,  107 
Arc,  cerebellar,  90 
cerebral,  89 
spinal,  89 
Areolar  tissue,  30 
Arginin,  10 
Arteries,  209 

pressure  in,    227,  245,  258,   259 

et  seq. 
Arterioles,  methods  of  studying  their 

condition,  260 
normal  state  of,  261 
walls  (muscular)  of,  262 


429 


430 


INDEX 


Arytenoid  cartilages,  307,  332 

Ash  of  milk,  319 

Asphyxia,  306 

Aspirates  (see  Consonants) 

Assimilation,  382 

Association,  mechanism  cerebral,  183 

Astigmatism,  115 

Atropin,  385 

effect  on  heart,  239 

effect  on  salivary  secretion,  330 

effect  on  sweat  secretion,  409 
Atwater  on  energy  requirements,  378 
Auditory  centre,  181 

nerve,  134  et  seq. 

cochlear  root  of,  134 

vestibular  root  of,  135 
Auerbach's  plexus,  328 
Auricle,  211 

musculature  of,  211 

pressure  in,  225 

Auriculo-ventricular  valves,  228 
Availability  of  food-stuffs,  381 
Axon,  76 

BACILLUS  coli  communis,  351 
Bacterial  action  in   the   alimentary 

canal,  350 

Bananas  as  food,  322 
Barcroft  and  Haldane's  method  for 
estimating  gases  in  the  blood, 
199 

Barfoed's  solution,  316 
Barium  salts,  influence  on  blood  pres- 
sure, 260 

Barnard  Hill's  sphygmometer,  258 
Barotaxis,  16 
Bases,  hexone,  10 

of  the  urine,  402 
Basilar  membrane,  132 
Basis  bundles,  153 
Basophil  leucocytes  (see  Leucocytes) 
Beans  as  food,  321 
Beaumont's  work  on  gastric  digestion, 

333 

Benzene  compounds,  423 
Benzoates  in  the  circulation,  399 
Bi-amide  of  carbonic  acid  (see  Urea) 
Bidder's  ganglion,  238 
Bile,  341  et  seq. 
Bile,  action  of  salts  of,  342 

as  a  haemolytic  agent,  342 

crystalline,  342 

duct,  326 

effect  of  drugs  on,  345 

flow  of,  344 

influence  of  nerves  on  flow  of, 
345 

mode  of  formation  of,  345 

nature  of,  345 

pigments,  342 

pigments,  fate  of,  353 


Bile,  pressure  under  which  secreted, 

345 
Biliary  calculi,  343 

colic  (see  Colic) 
Bilirubin,  198,  342,  370 
Biliverdin,  342,  353 
Binocular  vision,  123 
Biuret  test,  347 
Bladder,  urinary,  407 
Blastoderm,  416 
Blind  spot,  116,  181 
Blood,  187  et  seq. 

coagulation  of,  188 

constituents,  fate  of,  204 

constituents,  source  of,  201 

defibrinated,  188 

distribution  of,  204 

flow,  270 

flow  in  arteries,  273 

flow  in  capillaries,  273 

flow  in  veins,  273 

gases,  199 

plasma,  187 

platelets,  187,  193 

serum,  188 

specific  gravity,  188 

sugar,  316 

total  amount  in  body,  203 
Blood  corpuscles  (see  Leucocytes,  Ac.), 

192  et  seq. 
Blood-pressure,  245  et  seq. 

general  distribution,  245 

in  kidney,  405 

mean,  257 

method  of  measuring,  257 

method  of  measuring  in  man,  2;18 

respiratory  changes  in,  256,  29(1 

tracing,  256 

variations  in,  246 
Blood  vessels,  circulation  in,  IM:; 

coats  of,  244 
Bolus  of  food,  331 
Bone,  34  et  seq. 

chemistry  of,  38 

development,  intra-cartilaginous, 
35 

development,  intra-membranous, 
34 

marrow,  202 

structure  of,  34  et  seq. 
Bowman's  capsule,  403 

glands,  140 

Brain  (see  Cerebrum,  &c.) 
Breath  sounds,  288 
Broca's  convolution,  186 
Brodie,  on  extract  of  suprarenals,  385 
Bronchial  sound,  288 
Brown-Sequard,  on  administration  of 

testicular  substance,  388 
Brown-Sequard,  removal  of  the  supra- 
renals, 384 


INDEX 


431 


Brunner's  glands,  326 
Buccinator  muscle,  331 
Bulbous  plants,  321 
Butter  as  food,  319 
Butyrin,  319 

CAFFEINE,  383 
Calamus  scriptorius,  266 
Calcification  of  cartilage,  37 
Calculi,  urinary,  398 
Calories,  314 

definition  of,  73 
Calorimeter,  314 

Camerer,  observations  on  diet,  377 
Canal,  Haversian,  37 
Canaliculi,  35 
Canals,  semicircular,  133 

physiology  of,  164 
Cancellous  tissue  of  bone,  35 
Cane  sugar,  317 
Capillaries,  209,  244 

pulse  in,  254 

pressure  in,  245,  259,  267  et  seq. 
Capronin,  319 
Capsule,  internal,  168 
Carpylin,  319 

Carbohydrates,  action  of  gastric  juice 
on,  336 

as  diet,  375 

as    they    leave    the    alimentary 
canal,  357 

definition  of,  316 

energy  value  of,  318 

requirement  of,  379 

tests  for,  316  et  seq. 

their  conversion  to  fat,  365 
Carbon  dioxide  in  blood,  200 

in  the  urine,  403 

Carbon  monoxide  haemoglobin,  197 
Carbonate  of  ammonia,  359 
Cardiac  contraction,  nature  of,  236 
Cardiac  cycle,  219 

duration  of  phases  of,  221 
Cardiac  end  of  stomach,  325 

failure,  268 

glands  of  stomach,  325 

impulse,  223,  233 

muscle,  67 

plexus,  superficial,  240 

branches   of   the   vagus,   239   et 

seq.,  267 

Cardiogram,  225,  231 
Cardiograph,  224 
Cardiometer  (Roy's),  234 
Cardio-pneumatic  movements,  297 

during  hibernation,  298 
Carrots  as  food,  321 
Cartilage,  33 

hyaline,  33 

parenchymatous,  33 
Cartilages  of  the  larynx,  307 


Casein  (see  Milk) 
Caseinogen,  319,  411 
Catalysis,  7 
Cathelectrotonus,  48 
Catheterisation  of  lungs,  302 
Caudate  nucleus,  169 
Cells,  chalice,  24 

fat,  31 

nerve,  76 

pigment,  32 

reproduction  of,  18 
Cellulose,  320 
Central  spot  of  the  eye,  181 
Centre,  auditory,  181 

for  dilator  pupillae,  112 

for  smell,  140,  182 

for  speech,  185 

for  sphincter  pupillae,  112 

for  vomiting,  341 

respiratory,  290 

taste,  182 

touch,  182,  183 

vaso-constrictor,  265 

visual,  128 

voluntary,  184 
Centrosome,  14 
Cereals,  321 
Cerebellar  tracts,  152 
Cerebellum,  160  et  seq. 

connections  of,  162 

cortex  of,  161 

functions  of,  162 

grey  matter  of,  161 

peduncles  of,  155,  162 

removal  of,  162 

superior  vermis  of,  156 
Cerebral  cortex,  156,  169 

action  of,  174 

action  of  drugs  on,  175 

action,  time  of,  175 
Cerebrum,  168  et  seq. 

differentiation  of  stimuli  of,  174 

functions  of,  171 

grey  matter  of,  168 

localisation  of  functions,  178 

peduncles  of,  167 

removal  of,  171 
Cheese  as  food,  319 
Chemiotaxis,  16 
Chest  voice  (see  Voice) 
Chittenden  on  proteid  requirements, 

379 
Chlorine-containing  bodies  in  urine. 

402 

Cholalic  acid,  342 
Cholera  bacillus,  action    of    gastric 

juice  on,  336 

Cholesterin,  79, 194, 342  et  seq.,  353,  410 
Cholin,  78,  79 
Chondroitin,  33 
Chorda  tympani,  264,  330 


432 


INDEX 


Chordae  tendinse,  213,  215 
Chorionic  villi,  417 
Choroid,  107 
Chromatin,  18 
Chyle,  207 
Chyme,  341 
Cilia,  16,  27 
Ciliary  muscle,  107 

processes,  107 
Cilio-spinal  region,  113 
Circulation,  the,  209  et  seq. 

factors,  extra-cardiac,  in,  274etseq. 

foetal,  418 

in  blood  vessels,  244 

in  kidney,  403 

in  the  heart  wall,  274 

inside  the  cranium,  273 
Circulation  in  the  lungs,  274 

respiration,  influence  on,  295 
Claude  Bernard — formation  of  sugar 

in  the  liver,  366 
Coagulation  of  the  blood,  188  et  seq 

advantages  of,  190 

factors,  influencing,  189 

of  milk,  319  (see  also  Souring  of 

milk) 

Cochlea,  132 
Cocoa  as  food,  383 
Coffee  as  food,  383 
Cold  spots,  102 
Colic,  biliary,  343 
Collagen,  29 

action  of  hydrochloric  acid  on, 
335 

action  of  trypsin  on,  347 
Collodion  as  a  means  of  separating 

trypsin,  347 
Colloids,  10 
Colon,  326 
Colostrum,  411 
Colour,  104 

blindness,  122 

sensation  of,  119 
Colours,  compleruental,  121 
Columnae  carneae,  213 
Complement,  392 
Complemental  air,  286 
Conducting  paths  (sec  Spinal  cord,  &c.) 
Conduction  of  heat  from  skin,  361 
Conduction  of  nerve  impulse,  81  et  seq. 
Conductivity,  thermal,  101 
Connective  tissues  (see  Tissues) 
Consciousness,  93,  174 
Consonant  sounds.  311 
Contraction  of  the  pupil,  112 
Convection  of  heat  from  skin,  361 
Convoluted  stage  of  mitosis,  20 
Convoluted  tubules  of  the  kidney,  403 
Convolutions,  cerebral,  171 

left  inferior  frontal,  185 
Cooking,  effects  of,  on  food,  322 


Cooking,  methods  of,  322 

of  vegetables,  323 
Co-operative  antagonism  of  groups  of 

muscles,  60 

Cord,  spinal  (see  Spinal  cord) 
Cornea,  107 
Corona  radiata,  169 
Coronary  arteries,  231 
Corpora  quadrigemina,  167 
Corpus  Arantii,  216 

Highmori,  414 
Corpuscles,  blood,  192  et  sf<i. 

bone,  35 

muscle,  41 

nerve,  78 

tactile,  98 
Coiti,  organ  of,  134 
Coughing,  285 
Cranial  nerves,  157  et  seq. 
Cream  as  food,  319 
Creatin,  43,  191,  320,  370,  398 
Creatinin,  370,  398 
Cretinism,  388 
Cricoid  cartilage,  307 
Crossed  pyramidal  tract,  153 
Crura  cerebri,  162,  167  et  seq. 

<  rusta  of,  156,  167 
Cuneate  lobe,  181 
Curare,  experiment,  46 
Current  of  action,  66 

injury,  65 

Cutaneous  nerves,  influence  on  res- 
piration, 294 
Cyrtometer,  280 
Cystin,  401 
Cytomitoma,  14 
Cytotoxins,  392 

DECUSSATION  of  the  fillet,  155 

of  the  pyramids,  156 

of  the  optic  nerves,  128 
Defalcation,  355 
Degeneration,  Nissl's,  87 

reaction  of,  51 

Deiters'  nucleus,  153,  155,  157,  162 
Delivery,  419 
Dendrites,  76 
Dentals  (see  Consonants) 
Dentatenucleusof  the  cerebellum,  161 
Depressor  nerve  (see  Cardiac  branches 

of  vagus) 

Depth  of  sleep,  177 
Descending  antero-lateral  tract.  157 
Deutero-proteose,  12,  335,  346 
Development,  415  ft  ser/. 
Developmental  method   of    staining 

nerve  tracts,  153 
Deviation  of  eyes,  126 
Dextrin,  317,  329 
Dextrose,  316 
Diabetes,  386,  390 


INDEX 


433 


Diamido  acids,  10,  350,  424 
Diapedesis,  193 
Diaphragm,  281 
Diaphysis,  38 

Diastole  of  heart,  219,  221 
Dicrotic  notch,  251 

wave,  250,  251 
Diet,  375  et  seq. 

during  muscular  work,  72 

typical,    for    an    average    man, 

380 

Dietetics,  376 
Diffusion  of  gases  in  lungs,  287,  300, 

303 
Digestion,  312  et  seq. 

fate  of  the  secretions  of,  352 

intestinal,  341 

in  mouth,  328 

in  stomach,  333 

of  stomach  wall,  336 
Dilatores  narium,  284 

pupillse,  107 

pupillse,  course  of  fibres,  113 
Dioptric  mechanism,  106 
Dioxybenzene,  401 
Diphtheria  bacillus,  391 

toxin,  391 

Direct  or  dorsal  cerebellar  tract,  153, 
156,  162 

pyramidal  tract,  153 
Disaccharids,  317 
Discharging    mechanism    (cerebral), 

183 
Distinction  of  differences  of  pressure, 

99 
Distinction  of  place  of  contact,  100 

of  time  contacts,  100 
Diureides,  397,  425 
Divalent  acids,  423 
Dobie's  line,  41 
Ductless  glands,  384  et  seq. 
Ductus  arteriosus,  418 

arteriosus,  closure  of,  419 

venosus,  418 

venosus,  obliteration  of,  419 
Duodenum,  326 
Dyaster  stage  of  mitosis,  20 
Dynamometer,  55 

EAR,  anatomy  of,  130  et  seq. 

external,  130 

internal,  132  et  seq. 

middle,  130  et  seq. 
Eck's  fistula,  370 
Eggs  as  food,  320 
Ehrlich's  side  chain  theory,  391 
Elastin,  29,  315 

action  of  trypsin  on,  347 
Electrode,  non-polarisable,  65 
Electrotonus,  48 
Eleventh  nerve  (see  Spinal  accessory) 


Emmetropia,  110 

Emulsification,  347 

Endocardium,  214 

Endolymph,  166 

Endothelium,  30 

Energy  requirements  of  individuals, 

377 

factors  modifying,  376 
Energy  value  of  food,  313 
determination  of,  314 
Entero-hsemal  circulation,  352 
Enterokinase,  348,  350 
Enzymes,  7 

Eosinophiles  (see  Leucocytes) 
Epiblast,  416 

parts  developed  from,  416 
Epiglottis,  307 
Epiphysis,  38 
Epithelium,  ciliated,  27 
columnar,  23 
excreting,  27 
glandular,  25 
mucin-secreting,  25 
simple  squamous,  22 
stratified  squamous,  22 
zymin-secreting,  26 
Erection,  415 

mechanism  of,  415 
Erepsin,  350,  369 
Ergograph,  64 

Erythrocytes,  187,  194  et  seq.,  202 
chemistry  of,  194 
fate  of,  204 
Erythro-dextrin,  329 
Estimation    of    weight    of    objects, 

97 

Ethane,  421 

Ethereal  sulphates  in  urine,  399 
Ethylene,  421 
Euglobulin,  191 
Eustachian  tube,  131,  132 
Excretion,  effect  of   muscular  work 

on,  71 

by  the  kidneys,  394  et  seq. 
by  the  lungs  (see  Respiration) 
by  the  skin,  408 
Exophthalmic  goitre,  388 
Expiration,  284 
forced,  285 
Expired  air,  299 
Eye,  103  et  seq. 

anatomy  of,  106  et  seq. 
connection  with  central  nervous 

system,  126 

distance  of  central  spot,  117 
emmetropic,  110 
hyaloid  membrane  of,  108 
nervous  mechanism  of  movement, 

125 

physiology  of,  109 
Eyeballs,  movement  of,  124 

28 


434 


INDEX 


FACIAL  nerve,  159 
F«3ces,  353 

in  fasting  animals,  353 
in  feeding  animals,  353 
in  infants,  353 
Fallopian  tube,  414,  415 
Falsetto  voice  (see  Voice) 
Far  point  of  vision,  112 
Fasting,  metabolism  during,  373 
Fat  cells,  31 
Fatigue  of  cerebral  mechanism,  176 

of  muscle,  55 
Fats,  31,  191 

in  bacon,  320 

in  diet,  376 

energy  value  of,  315 

gastric  juice  on,  336 

metabolism  of,  372 

as    they    leave    the    alimentary 

canal,  357 
Fatty  acids,  31 

tissue,  storage  in,  365 
Feeding,  effect  of,  374 
Fehling's  test,  316 
Fenestra  ovalis,  130 

rotunda,  130 
Ferrier,    experiments    on    brain    of 

monkey,  181,  182 
Ferro-proteids,  13 
Fibres,  elastic,  29 
nerve,  78 
non-elastic,  29 
non-medullated  nerve,  78 
pre-ganglionic  and  post-gangli- 

onic  fibres,  143 
splanchnic,  144  et  seq. 
somatic,  144 
Fibrin,  189 
Fibrinogen,  189 
Fibroblasts,  28 
Fibro-cartilage,  elastic,  34 

white,  34 
Fibrous  tissue,  28 
Field  of  vision,  117 
Fifth  cranial  nerve  (see  Trigeminal) 
Filiform  papillae,  324 
Fillet,  155  et  seq. 
Fissure  of  Rolando,  183 
Flesh,  319 

effect  of  cooking  on,  322 
preserved,  320 
Flocculus,  161 

Floor  of  the  fourth  ventricle,  155 
Fluoride  of  sodium,  action  on  epithe- 
lium, 358 

Foetal  circulation,  418 
Food,  312  et  scq. 

absorption  of,  340  et  seq. 
animal,  318 

effect  of,   on  respiratory  inter- 
change, 305 


Food,  fate  of  absorbed,  360 

nature  of,  312 

vegetable,  320 
Foramen  ovale,  418 

occlusion  of,  419 
Formamide.  \'l  1 
Formic  acid,  424 
Foster,  changes  in  protoplasm,  8 
Fourth  cranial  nerve  (»cc  Trochlearis) 
Frauenhofer's  lines,  I'.t.'j 
Frog's  heart,  219 
Functions  of  the  spinal  cord,  149 

GALACTOSE,  316 

Gall-bladder,  326 

Gall-stones,  343 

Galvanic     current,     stimulation     of 

muscle  by,  50 
Galvanotaxis,  16 
Galvanotonus,  47 
Ganglia,  collateral,  143 

inferior  mesenteric,  144 

lateral,  144 

superior  cervical,  144 

superior  meseuteric,  144 
Gases  of  the  blood,  199 
Gaskell,  nervous  mechanism  of  the 

heart,  241 

Gastric  digestion,  amylolytic  period, 
333 

proteolytic  period,  334 
Gastric  glands,  325 
Gastric  juice,  333 

action  on  gelatin,  335 

antiseptic  action,  336 

influence  of  diet  on,  337 

source  of  constituents,  336 
Gelatin,  29,  315 

action  of  trypsin  on,  347 
Gemmules,  77,  175 
Generative  organs,  413  et  seq. 
Geniculate  bodies,  1M 
Germ  centres,  201 
Germinal  epithelium,  414 

spot,  414 

vesicle,  414 
Gestation,  419 
Giant  cells,  202 
Glands,  compound,  25 

racemose,  25 

simple  tubular,  25 
Globin.  11)9 
Globulin,  11 
Globulose,  11 

Glomerulus  of  kidney.  403 
Glossopharyngeal  nerve,  158,  331 

influence  on  respiration,  293 
Glucosamine,  26,  33 
Glucose,  42,  191,  313 

of  blood,  201 
Glycerin,  422 


INDEX 


435 


Glycero-phosphates  in  urine,  402 

Glycocholic  acid,  341 

Glycocoll,  399 

Glycogen,  42,  318 

Glycogenic  function  of  the  liver  (see 
Liver) 

Glyco-proteids,  13,  26 

Glycosuria,  368 

Glycuronic  acid,  33 

Gmelin's  test,  342,  353 

Golgi,  organs  of,  97 

Golgi's  method  of  nerve  staining,  170 

Goltz,  experiments  on  cerebral  cortex, 
172 

Graafian  follicle,  413 

Granules,  Nissl's,  76 

Gravity,  influence  on  capillary  pres- 
sure, 269 

Grey  matter  (see  Cerebrum,  &c.) 

Growth  in  length  of  bone,  38 

Grubler's  peptone,  357 

Guanidin,  43 

Gutturals  (see  Consonants) 

H^MATIN,  198 
Hsematoidin,  198,  343 
Hsematoporphyrin,  198,  343,  402 
Haemin,  198 
Hasmochromogen,  198 
Haemocytometer,  194 
Haemoglobin,  43,  195 

action  of  hydrochloric  acid  on, 
335 

decomposition  of,  198 

reduced,  195 

Hsemoglobinometer,  Haldane's,  196 
Haemolymph  glands,  201,  205 
Haemolysis,  205,  393 
Hair,  408 

Haldane,  experiments  on  air  contain- 
ing carbon  monoxide,  302 

and  Barcroft,  method  for  gases 
in  blood,  199 

and  Lorrain  Smith,  method  for 

total  blood,  203 
Hassall,  corpuscles  of,  389 
Haversian  canals,  spaces,  &c.,  37 
Hearing,  129  et  seq. 
Heart,  211  et  seq. 

changes  in  shape,  222 

changes  in  position,  222 

connection  with  central  nervous 
system,  239  et  seq. 

connection  in  frog,  239 

connection  in  mammal,  239 

effect  of  atropin  on,  239 

electrical  stimulation  of,  239 

fibrous  rings  of,  212 

of  frog,  219 

physiology  of,  219 

pulmonic,  210,  211 


Heart,  relations  of,  218 

sounds  of,  231  et  seq. 

structure,  211  et  seq. 

systemic,  209,  211 

valves  (see  Valves) 

work  of,  234 
Heat  elimination,  361,  364 

elimination,  nervous  mechanism 
in,  365 

elimination  from  skin,  361 

elimination  from  respiratory  pas- 
sages, 362 

elimination  in  urine  and  faeces,  362 

production,  360,  364 

production  in  the  brain,  361 

production  in  glands,  360 

production  in  muscle,  360 

units,  314. 
Henle's  loop,  403 
Hemisection  of  spinal  cord,  151 
Hemispheres,  cerebral  (see  Cerebrum) 
Herbivorous  animals,  313 
Hetero-proteose,  334 
Hexone  bases,  10 
Hiccough,  286 
High  tension  pulse,  254 
Highmori,  corpus  (see  Corpus) 
Hippocampal  convolution,  182 
Hippuric  acid,  399 
Histidin,  10 
Histones,  12 

Holoblastic  segmentation,  415 
Hoppe-Seyler,     changes     in     proto- 
plasm, 8 
Hot  spots,  102 
Hyaline  cartilage,  33 
Hyaloid  membrane,  108 
Hyaloplasm,  15 
Hydrocarbons,  unsaturated,  421 

saturated,  421 
Hydrochloric  acid,  334 
Hydroxyl  molecule,  422 
Hypermetropia,  115 
Hypnosis,  177 
Hypoblast,  416 

parts  developed  from,  416 
Hypoglossal  nerve,  158 
Hypoxanthin,  398 

ILEO-C^ECAL  valve,  326 
Illumination,  degree  of,  103 

source  of,  103 
Illusions,  optical,  129 
Immune  body,  392 
Immunity,  391 
Impregnation,  415 
Incompetence  (cardiac),  234 
Incus,  131 
Indol,  351,  354,  399 
Indoxyl,  400 
Indican,  400 


436 


INDEX 


Induced  electricity,   stimulation   of 

muscle  by,  47 
Inferior  oblique  muscle,  125 

olivary  nucleus,  155 
Inflation  of  lungs,  artificial,  292 
Infundibula,  pulmonary,  215,  278 
Inhibition,  intra-cardiac  mechanism. 

238 

Inhibitory  action  of  vagus,  241 
Inhibitory  nerves,  85 
Inorganic  salts  in  food,  312 
Inosite,  43,  320 
Inspiration,  280 
forced,  284 
forces  opposing,  281 
Intercostal  muscles,  external,  283 
Internal  capsule.  168 
Internal  secretion,  384 

secretion,  effect  on  blood  pres- 
sure, 260 
Intestines,  325  et  set]. 

absorption  from,  358  et  seq. 
digestion  in,  341  et  seq. 
movements  of,  354 
nerve  supply  of,  327 
secretion    of    wall    (tee    Succus 

entericus) 
Intra-cardiac  pressure,  changes  in, 

225 
pressure,  method  of  determining, 

225 

Inulin,  317 
lodothyrin,  386 
Iris,  107 

Islets  of  Langerhans,  327 
Isometric  method  of  measuring  mus- 
cular contraction,  55 
Isotonic  method  of  measuring  mus- 
cular contraction,  55 

JACKSONIAN  epilepsy,  174,  182,  185 
Jacobsen's  nerve,  33i 
Jecorin,  191 
Joule's  law,  314 
Juice,  gastric,  333  et  seq. 
pancreatic,  346 

KATABOLIC  changes,  8 

Katacrotic  crests,  252 

Kennedy's     experiments     on    nerve 

crossing,  81 
Keratin,  13,  23,  315 
Ketones,  422 
Knee  jerk,  151 
Kidney,  403  et  seq. 

circulation  in,  403 

physiology,  404 

structure,  403 

LABIALS  (see  Consonants) 
Labour,  419 


Labyrinth  of  ear,  132 
Lactase,  350 
Lactate  of  ammonia,  370 
Lacteals,  358 
Lactic  acid,  412,  423 
Lactose,  317,  319 
Lacunae,  35 
Laevulose,  316 
Lamellae,  Haversian,  37 

interstitial,  37 

medullary,  37 

peripheral,  37 

Langerhans,  islets  of,  327,  390 
Langley,  experiment  on   vagus  and 

sympathetic  nerves,  81 
Language,  spoken,  310 
Lanolins,  410 
Large  intestine,  326 
Laryngoscope,  308 
Larynx,  cartilages  of,  307 

mucous  membrane  of,  308 

muscles  of,  308 

nervous  mechanism  of,  309 

physiology  of,  309 

position  in  deglutition,  332 

structure  of,  307 
Lateral  fillet.  I", 
Law,  Pfliiger's,  83 

of  polar  excitation,  49 
Lecithin,  78,194, 320, 343, 366, 368, 412 
Left  inferior  frontal  convolution,  185 
Legumens,  321 
Lens,  crystalline,  108 
Lenticular  nucleus,  168 
Lentils,  as  food,  321 
Lesions  of  the  cerebral  cortex,  185 
Leucin,  11,  347,  350 
Leucoblasts,  202 
Leucocytes,  187,  192  et  seq.,  201 

chemistry  of,  193 

fate  of,  204 
Leucocytosis,  30,  202 

digestion,  358 
Levatores  costarum,  283 
Levy  and    Zuntz's    experiments    or 

diet,  374 

Lieberkiihn's  follicles,  325,  326,  349 
Light,  103 
Liquor  folliculi,  414 

sanguinis,  187 
Lipochrome,  190 
Lissauer's  tract,  153 
Liver,  205,  326  ct  seq. 

development  of,  366 

formation  of  urea  in,  369 

glycogenic  function,  366 

relation  to  fats. 

relation  to  general  metabolism, 
366 

relation  to  proteids,  368 

storage  of  carbohydrates,  366 


INDEX 


437 


Liver,  summary  of  functions,  373 
Localisation  of  functions,  178 
Lockhart  Clarke's  column,  90,  153 
Lorrain  Smith  and  Haldane's  method 

for  total  blood,  203 
Loudness  of  sounds,  309 
Lungs,  278,  279 

air  sacs,  278 

capillaries,  278 

elasticity  of,  278 

interchange    between     air    and 
blood  in,  298 

physiology  of,  279  et  seq. 
Lymph,  187  et  scq.,  206  et  seq. 

chemistry  of,  207 

in  disease,  207 

formation  of,  208 

specific  gravity  of,  207 

vessels,  209 

Lymphatic  glands,  201,  209,  258 
Lymphatics,  pressure  in,  246,  270 
Lymphocytes,  192 
Lymphoid  tissue,  30,  201 
Lysin,  10 

MACULA  lutea,  181 
Maize,  321 
Malic  acid,  313 
Malleus,  131 
Malpighian  bodies,  403 

bodies,  secretion  in,  404 

corpuscles,  206 
Maltose,  317,  329 
Mammary  glands,  409,  411 
Marchi's  method   of    nerve-staining, 

86,  152 

Mariotte's  experiment,  116 
Mastication,  328 

Maternal  attachment  of  ovum,  417 
M'Fie,  experiments  with  extract  of 

suprarenal,  385 
Meconium,  354 
Medulla  of  kidney,  403 
Medulla  oblongata,  154  et  seq. 

commissural  fibres  of,  157 

conducting  paths  in,  155 

grey  matter  of,  155 

pyramids  of,  156 

reflexes  of,  160 

structure  of,  154 
Medullary  sheath,  78 
Meissners  plexus,  328 
Melanin,  32 

Membrana  tympani,  130 
Membranous  labyrinth,  133 
Memory,  175 
Menstrual  period,  415 
Menstruation,  414 
Mesial  fillet,  156 
Mesoblast,  416 

parts  developed  from,  416 


Metabolic  equilibrium,  374 
Metabolism,  general,  8,  371 

during  fasting,  373 

of  fats,  372 

proteid,  371 

method  of  investigating,  371 
Methsemoglobin,  196 
Methane,  421 
Methylene  blue,  use  in  muscle  ex- 

"  periments,  303 
Micro-organisms,  193 

in  cheese,  319 
Micturition,  407 
Milk,  318 

composition  of  cow's,  318 

composition  of  human,  318 

effect  of  cooking  on,  322 

fats,  412 

phosphorus  compounds  of,  412 

proteids  of,  411 

secretion  of,  410 

souring  of,  412 

sugar  of,  412 
Mitosis,  19 
Mitral  area,  233 

valve,  214 

Molecular  layers  of  retina,  108 
Monamido  acids,  11,  424 
Monaster  stage  of  cell  division,  20 
Monosaccharids,  316 
Monotremes,  182 
Motor  nerves  (see  Nerves) 

points,  46 

Mott,  views  on  the  Kolandic  area,  183 
Mouth,  324 

nerve  supply  of,  327 
Mucin,  13,  25,  26,  315,  344,  387 

secreting  epithelium,  25 
Mucoid  tissue,  27 

Mucous  membrane  of  large  intestine, 
326 

of  small  intestine,  325 

of  stomach,  325 

of  uterus,  414 
Murexide  test,  398 
Murmurs,  cardiac,  233 

simulation  of,  by  breath  sounds, 

298 
Muscle,  40  et  seq. 

absolute  force  of,  55 

application  of  weights  to,  57 

carbohydrates  of,  42 

cardiac,  67 

changes  in  blood  passingthrough, 
69 

changes  in  shape,  51 

chemical  changes  in,  67 

chemistry  of,  41 

death  of,  74 

duration  of  contraction,  54 

electrical  changes  in,  65 


438 


INDEX 


Muscle,  electrical  condition  of,  45 

extensibility  of,  44,  66 

fatigue,  55 

heat  production  in,  45,  63 

latent  period  of  contraction,  54 

minimum  stimulus,  ;"f. 

optimum  load,  62 

optimum  stimulus,  57 

physical  characters  of,  44 

proteids  of,  42 

respiration  of  excised  muscle,  69 

skeletal,  40 

spindles,  97 

stimulation  by  galvanic  current, 
50 

stimulation    by    induced     elec- 
tricity, 49 

storage  of  food  in,  366 

structure  of,  40 

successive  stimuli,  57 

temperature  effects,  56 

visceral,  40,  66 

visceral,  staircase  contraction  of, 
67 

voluntary  contraction,  59 

wave  of  contraction,  52 

work,  61 
Muscles,  co-operative  antagonism  of, 

60 
Muscular  sense  (see  Sense) 

sense,  centre  for,  183 
Muscular  work,  effect  on  excreta,  71 

work,  man's,  for  a  day,  378 
Myelocytes,  193 
Myogenic   movements  of    intestine, 

354 

Myoglobulin,  42 
Myohacruatin,  43 
Myoids,  16,  40 
Myopia,  114 
Myosin,  42 
Myosinogen,  42 
Myostromiii,  l_ 
Myxoedema,  388 

NASAL  cavity,  139 

Nasals  (see  Consonants) 

Naunyn's  observations  on  cholesterin, 

343 

Near  point  of  vision,  112 
Necrobiosis,  8 
Nerve  cells,  76 

cells,  function  of,  86 

centres  (see  Centres) 

corpuscles,  78 

fibres,  78 

fibres,  medullary  sheaths  of,  78 

fibres,  non-medullated,  78 

specific  energy  of,  104 
Nerves,  75  et  seq. 

afferent,  85 


Nerves,  auditory,  135  ;  cochlear  root 
of,  136  ;  vestibular  root  of,  137 

augmentor,  85 

efferent,  84 

excito-motor,  85 

excito-reflex,  85 

excito-secretory,  85 

factors  modifying  conduction  in, 
83 

inhibitory,  85 

mixed,  85 

motor,  85 

nature  of  impulse  in,  86 

physiology  of,  86 

rate  of  conduction  in,  82 

secretory,  85 

sensory,  85 

spinal  (see  Spinal  nerves) 

vaso-constrictor,  85,  263,  264 

vaso-dilator,  264 

Nervi  erigentes,  146,  264,  355,  407 
Nervous  mechanism,  intra-cardiac,  238 

system,  141  et  seq. 
Neural  canal,  41»> 
Neurilerama,  78 
Neuroglia,  148,  416 
Neuro-keratin,  78 
Neuro-muscular  mechanism,  88  et  seq 

fatigue  of,  94 

Neutral  sulphur  in  urine,  401 
Neurons,  76,  416 

classification  of,  84 

excitability  of,  80 

extent  of  excitability  of,  88 

ingoing,  81 

manifestation  of  activity  of,  81 

outgoing,  81 

stimulation  of,  80 
Nicotine,  effect  of  painting  ganglia 

with,  143,  330 
Ninth  nerve,  146,  158 
Nissl's  degeneration,  87 

granules,  76,  87,  176 
Nitrites,  effect  on  blood  pressure,  260 
Nitrogen  in  the  blood,  201 

in  the  urine,  396  et  m>/. 
Nodal  swellings,  18 
Nodes  of  Ranvier,  78 
Nuclei  of  the  cranial  nerves,  158 

of  the  posterior  columns,  89 
Nucleins,  12 

Nucleo-albumin  in  the  urine,  402 
Nucleolus,  17 
Nucleo-proteids,  344 

action  of  gastric  juice  on,  335 

action  of  trypsin  on,  347 
Nucleus,  17  et  seq. 

cuneatus,  152,  155 

gracilis,  152,  166 
Nutrition  of  the  brain,  173 
Nuts,  as  food,  322 


INDEX 


439 


OATMEAL,  as  a  food,  321 
Occipital  lobe,  181 
Oculomotor  nerve,  159 
(Esophagus,  325 

application  of  a  stethoscope  to, 
332 

peristaltic  action  of,  332 
Oleic  acid,  31 
Olein,  32,  319 
Olfactory  bulb,  139 

tracts,  139 
Olive,  the,  155 
Ophthalmoscope,  109 
Optic  chiasma,  126 

disc,  108 

nerve,  108 

radiation,  181 

thalamus  (see  Thalamus) 
Ora  serrata,  109 
Organ  of  Corti,  134 
Organic    acids    as    salts     in     food, 

313 

Organs  of  Golgi,  97 
Osazones,  316 
Osmosis,  358 
Osmotic  pressure,  194 
Ossification,  process  of,  34  ct  seq. 

centre  of,  36 
Otic  ganglion,  331 
Ovary,  388,  414 
Ovum,  413 
Oxalates,  189 
Oxalic  acid,  403,  423 
Oxy-acids,  423 
Oxygen  in  blood,  200 
Oxyhasmoglobin,  195 
Oxyntic  cells,  325 
Oxyphil  leucocytes,  192 

PAIN,  sensation  of,  96 
Palmitic  acid,  31 
Palmitin,  32,  319 
Pancreas,  327,  390 

removal  of,  368 
Pancreatic  secretion,  346 

secretion,  action  of,  346 

secretion,  character  of,  346 

physiology  of,  349 
Papillary  muscles,  213 
Paracasein  calcic.  319,  336,  411 
Paradoxical  contraction,  84 
Paralysis  of  laryngeal  nerves,  309 
Paramyosinogen,  42 
Parathyroid,  388 
Parotid  gland,  324 

nerve  supply,  329 

reflex  stimulation  of,  331 
Partial  pressure  of  oxygen  and  carbon 
dioxide  in  blood,  301 

pressure  of  gases  in  air  vesicle, 
301 


Passage  of  carbon  dioxide  from  tis- 
sues to  blood,  304 

of  oxygen  from  blood  to  tissues, 

303 

Pawlow,  effect  of  diet  on  gastric  juice, 
337 

pancreatic  fistulae,  348 
Peas  as  food,  321 
Penis,  415 
Pepsin,  75,  337 

fate  of,  352 

source  of,  336 
Pepsin ogen,  337 
Peptic  glands,  325 
Peptone,  12,  189,  335,  346 
Perceptions,  modified,  129 
Perfusion  method  of  studying  action 

of  drugs,  261 
Pericardium,  218 
Perichondrium,  36 
Perilymph,  133,  166 
Perimeter,  117 
Perineurium,  79 
Peripheral  resistance,  259 
Peristalsis  of  bladder  wall,  408 

of  intestine,  354 

nervous  mechanism  of,  355 

of  wall  of  ureter,  407 

wave  of,  67 

Petrosal  nerve,  small  superficial,  331 
Peyer's  patches,  326 
Pniiger's  experiments  on  proteid  diet, 
375 

law  of  contraction,  83 
Phacoscope,  112 
Phagocyte  action,  193 
Pharynx,  324 

nerve  supply  of,  327 
Phenol,  351,  399 
Phenylhydrazin,  316 
Phloridzin,  injection  of,  368 

poisoning,  367 
Phosphocarnic  acid,  42,  412 
Phospho-proteid,  320 
Phosphorus-containing  bodies  in  the 

urine,  401  et  seq. 
Phototaxis,  16 
Phrenic  nerves,  290 
Pialyn,  347 
Pigment  cells,  32 
Pigments  of  the  urine,  402 
Pillars  of  the  fauces,  332 
Pilocarpine,  effect  on  sweat  glands, 

410 

Pitch  of  sounds,  136,  309 
Pituitary  body,  386 

injection  of  extracts  of,  386 
Placenta,  417 
Plasma,  190  et  seq. 
Plasmodia,  4 
Plethysmograph,  261 


440 


INDEX 


Pleura,  complemental,  282 
Pleural  cavity,  278 
Plexus,  hypogastric,  146 

Auerbach's,  328 

Meissner's,  328 
Pohl's  observations  on  Leucocytosis, 

358 

Polar  excitation,  law  of,  49 
Polycrotic  waves,  250 

pulse,  252 
Polymerisation    of    raonosaccharids, 

317 
Polymorpho-nuclear    leucocytes    (see 

Leucocytes) 
Polysaccharids,  317 
Pons  Varolii,  155,  160 
Portal  circulation,  327 

vein,  327 
Positive  accommodation,  111  et  seq. 

accommodation,   varying  power 

of,  114 
Posterior  columns  of  spinal  cord,  152 

columns,  nuclei  of,  89 

corpora  quadrigernina,  166 
Potatoes,  as  food,  321 
Poteriodendron,  75 
Prsecordium,  218 
Precipitins,  393 
Predicrotic  wave,  250,  251 
Preformed  sulphate  in  urine,  399 
Preganglionic  fibres,  143 
Presbyopia,  114 
Pressure,  arterial,  259 

arterial,  factors  controlling,  259 
Primitive  nerve  sheath  (see  Schwann) 
Propionic  acid,  423 
Prostate  gland,  414 
Protagon,  78 
Protamines,  10 
Proteid  metabolism,  371 
Proteids,  9  et  seq. 

as    they    leave    the    alimentary 
canal,  357 

of  blood,  201 

of  cereals,  321 

conjugated,  12 

conversion  into  fat,  365 

diet,  375 

energy  value  of,  314 

native,  11 

requirements,  378 

sparers,  375 

Proteolytic  period  (see  Gastric  secre- 
tion) 

Proteoses,  11 
Prothrombin,  189 
Protoplasm,  4  et  seq. 

cell,  14 

Proto-proteoses,  12,  334 
Proximate  principles,  313 

sources  of,  318 


Pseudoglobulin,  191 
Pseudopodia,  193 
Pseudo-nucleins,  12 
Ptomaines,  352 
Ptyalin,  328 

fate  of,  352 
Puberty,  413 
Pulmonary  area,  232 

valve,  217 
Pulse,  anacrotic,  252 

arterial,  246 

capillary,  254 

cause  of,  246 

characters  of  wave,  247 

form  of  wave,  249,  254 

height  of  wave,  249 

high  tension,  254 

length  of  wave,  248 

palpation  of,  252 

rate  of,  253 

rate,  variations  in,  220 

rhythm,  253 

tension,  253 

velocity  of  wave,  248 

venous  (see  Venous  pulse) 

volume  of,  253 
Pulsus  celer,  254 

parvus,  253 

plenus.  253 

tardus,  254 
Pupil,  107 

contraction  of,  112 
Purin  bodies,  370,  372,  397 

bodies,    endogenous    and    exo- 
genous, 397 
Purkinje's  cells,  161 

images,  118 

Pyloric  end  of  the  stomach,  325 
Pyramidal  tracts,  153 
Pyramids  of  the  medulla,  156 
Pyrocatechin,  401 

QUADRIURATES,  398 

Quality  of  sounds,  310 

RADIATION  of  heat  from  skin,  361 
Radicals,  organic,  421 
Ranvier,  nodes  of,  78 
Reaction,  94 

of  degeneration,  51 
Receptaculum  chyli,  358 
Reception  of  stimuli,  178 
Recollection,  175 
Recti  muscles  (eye),  124,  125 
Rectum,  326 

Recurrent  laryngeal  nerve,  240 
Red  marrow  of  bone,  202,  206 
Red  nuclei,  162,  167 
Reduced  alkaline  hzematin,  198 
Reflex  action,  87,  91,  176 

function  of  the  spinal  cord,  149 


INDEX 


441 


Refraction  of  light,  110 
Reid  on  absorption,  358 
Relation  of  haemoglobin  and  its 

derivations,  199 
Remak's  ganglion,  238 
Rennet,  319,  411 
Rennin,  337 

source  of,  33G 
Reproduction,  413 
Reserve  air,  286 
Residual  air,  287 

contraction,  period  of  (see  Systole) 
Respiration,  277  ct  seq. 

effect  on  air  breathed  of,  298 

effect  on  the  blood,  299 

at  high  altitudes,  303 

influence  of   heart's   action  on, 
297 

internal,  303 

mechanism  of,  277  et  seq. 

movements  in  (special),  285 

nervous  mechanism  of,  289 

physiology  of,  279 

rhythm  of,  289 
Respiratory  centre,  290 

action  of,  291 

effect  of  absence  of  oxygen  on, 
294 

effect  of  accumulation  of  waste 
products  in  the  blood,  295 

effect  of  carbon  dioxide  on,  294 

effect  of  temperature  of  animal 
on,  295 

modifications     in     activity    of, 

294 

Respiratory  interchange,  extent  of, 
304 

interchange,  factors  modifying, 
304,  305 

quotient,  299 

quotient,  effect  of  diet  on,  305 
Restiform  bodies,  155 
Rete  testis,  414 
Retina,  108  et  seq. 

blood  vessels  of,  109 

corresponding  areas  on  the  two 
sides,  124 

nature  of  changes  in,  118 

stimulation  of,  116,  118 

rods  and  cones  of,  109 
Rhythmic  contraction  of  the  heart, 
236 

contraction,  propagation  of,  236 

contraction,  starting  mechanism 

of,  237 

Rigor  mortis,  74 
Rolandic  area,  183,  185 
Rolando  (see  Fissure) 
Roof  nucleus  of  the  cerebellum,  161 
Roots  of  the  spinal  nerves,  142 
Roy's  cardiometer,  234 


SACCHAKOMYCES  cerevisae,  4 
Saccule,  133 

St.  Martin  (see  Beaumont) 
Saliva,  328 

functions  of,  329 

influence  of   chorda  tympani  in 
secretion  of,  330 

physiology  of,  329 

reflex  stimulation   of  secretion, 
331 

stimulation  of  sympathetic,  effect 

on,  331 
Salivary  glands,  324  et  seq. 

glands,   nerve  supply,   327,  329 

et  seq. 

Salol,  rate  of  passage  through  intes- 
tine, 355 
Salts  of  the  bile  acids,  341,  352 

fate  of,  352 
Sanson's  images,  112 
Sarcolactic  acid,  43 
Sarcolemma,  40,  41 
Sarcous  substance,  41 
Scala  media,  133 
Schafer's    investigations     of     brain 

functions,  182,  183 
Schemer's  experiment,  111 
Schwann,  white  sheath  of,  78 
Sciatic  nerve,  264 
Sclerotic,  107 
Sebaceous  glands,  409-410 
Secretion,  391 

internal,  384  et  seq. 
Sectional  area  of  circulatory  system, 

210 

Semen,  414,  415 
Semicircular  canals,  132  et  seq. 

canals,  physiology  of,  164 
Semilunar  valves,  216 

valves,  action  of,  229 
Seminiferous  tubules,  414 
Sensation,  94 

colour,  119 

physiology  of  colour,  120 

production  of  colour,  121 

of  hunger,  95 

of  pain,  96 

of  smell,  139 

of  taste,  138 

of  thirst,  95 

visual,  129 
Sense  (The  Senses),  95  et  seq. 

of  acceleration  and  retardation 
of  motion,  164 

joint  and  muscle,  97  et  seq.,  164 

special,  98  et  seq. 

of  smell,  139 

tactile,  98 

of  taste,  138 

temperature,  101 

visual,  102  et  seq. 

29 


442 


INDEX 


Sensibility,  common,  95 
Septo-marginal  tract,  153 
Serum,  190  et  seq. 

albumin,  189,  191 

globulin,  189,  191 

Seventh  cranial  nerve,  146  (see  Facial) 
Sexual  organs,  development  of,  413 

organs,  removal  of,  413 
Side-chain  theory  (see  Ehrlich) 
Sighing,  286 
Singing  voice,  310 
Sinus  of  Valsalva,  216 
Sixth  nerve  (see  Abdncens) 
Skatol,  347,  354,  399 
Skatoxyl-sulphate  of  potassium,  400 
Skin,  excretion  by,  408 
Sleep,  177 

respiratory  changes  during,  305 

walking,  92,  177 
Small  intestine,  325 
Smell,  139 

centre  for,  182 

mechanism  of,  139 

physiology  of,  139 
Snake  toxin,  391 
Sneezing,  285 
Soluble  native  proteids,  346 
Somatopleur,  142,  416 
Sound  perception,  136 
Sounds  of  the  heart  (tee  Heart) 
Soup,  as  food,  323 
Space,  Haversian,  37 
Specific  nerve  energy,  104 
Spectrum  analysis,  195 
Speech  (spoken  language),  centre  for, 

185 

Spermatoblasts,  414 
Spermatogen,  414 
Spermatozoa,  413,  414 

development  of,  414 
Sphincter  an'.,  326,  335 

pupillse,  107 

trigonalis,  407 
Sphygmograph,  249 

tracings,  250 
Sphygmometer,  258 
Spinal  accessory  nerve,  15s 

cord,  148  et  seq. 

cord,  conducting  paths  in,  l.'l 

cord,  grey  matter  of,  148 

cord,  posterior  columns  of,  152 

cord,  reflex  function  of,  149 

cord,  section  of  one  side  of,  151 
"  Spinal  dog,"  91 
Spinal  nerves,  141 

nerves,  roots  of,  142 
Spirometer,  286 
Splanchnic  area,  274 

influence  on  respiration,  293 
Splanchnopleur,  142,  416 
Spleen,  205 


Spot,  blind,  116 

Staircase  contraction,  236 

Stannius'  experiment,  L'.'iT 

Stapedius  muscle,  131 

Stapes,  131 

Starches,  317 

Stearic  acid,  31 

Stearin,  32,  319 

Stenosis,  234 

Stercobilin,  353 

Stethoscope,  233 

Stewart's  method  of  estimating  time 

taken  in  circulation,  276 
Stewart  on  loss  of  heat,  378 
Stimulus,  94 
Stomach,  325 

during  fasting,  333 

during  feeding,  333 

importance  in  digestion,  340 

movements  of,  338 

nervous  mechanism  of,  339 

rate  of  passage  of  food   from, 
339 

vascular  changes,  333 
Storage  of  surplus  food,  365 
Storing  mechanism  (cerebral),  183 
Stromuhr,  272 
Sublingual  gland,  324 

gland,  nerve  supply,  329 
Submaxillary  gland,  324 

gland,  nerve  supply,  329 
Succi's  fast,  373 
Succus  entericus,  349 

entericus,  action  of,  349 

entericus,  nervous  mechanism  of 

secretion,  350 
Sulcus  centralis,  183 
Sulphur-containing  bodies  in  the 

urine,  399  et  seq. 
Superior  oblique  muscle,  125 
Suprarenal  bodies,  384 

extract  of,  263,  368 

medulla,  384 

metabolism   after   injection    of, 

385 

Suspensory  ligament  of  eye,  108 
Swallowing,  331 
Sweat,  chemistry  of,  410 

evaporation,  362 

glands,  409 

nervous  mechanism  of  secretion 

of,  409 
Sympathetic  fibres  to  the  heart,  240- 

243 

Synapsis,  76 
System,  Haversian,  37 
Systole  of  heart,  219,  221 

latent  period,  230 

period  of  overflow,  230 

period  of   residual   contraction, 
230 


INDEX 


443 


TACTILE  sense  (see  Sense) 
Tapetum  nigrum,  109 
Tartaric  acid,  313 
Taste,  137  et  seq. 

bulbs,  137 

centre  for,  182 

mechanism  of,  137 

physiology  of,  138 
Taurocholic  acid,  342 
Tea,  383 
Tegmentum,  167 
Temperature,  362  et  seq. 

in  cold-blooded  animals,  364 

in  hibernating  animals,  364 

in  warm-blooded  animals,  364 

mean  daily,  of  man,  363 

regulation,  361  et  seq. 

variations  in,  363 
Temporo-sphenoidal  lobe,  182 
Tension  of  gas  in  a  fluid,  301 

of  the  vocal  cords,  310 
Tensor  tympani  muscle,  131 
Tenth  nerve  (see  Vagus) 
Testis,  388,  414 
Tetanus,  complete,  59 

incomplete,  58 

Thalamus  opticus,  167,  168,  181 
Theine,  383 

Thermal  sense,  centre  for,  183 
Thermotaxis,  17 
Third  nerve,  146,  159 
Thoracic  breathing,  283 
Thrombin,  189 
Thymus  gland,  189,  389 

effect  of  castration  on,  390 

removal  of,  390 
Thyroid  cartilage,  307 

gland,  386 

administration  of  extracts  of,  387 

removal  of  (thyroidectomy),  387, 

388 

Tidal  air,  286 
Tissue,  adipose,  31 

areolar,  30 

cancellous,  35 

connective,  27  et  seq. 

fibrous,  28 

lymph,  30 

mucoid,  27 

vegetative,  23  et  seq. 
Tongue,  324 
Tonsil,  324 

Torricellian  vacuum,  199 
Touch,  centre  for,  182 
Toxic  action,  391  et  seq. 
Trabecula3,  36 
Tricrotic  pulse,  250 
Tricuspid  valve,  215 

area,  233 

Trigeminal  nerve,  159 
Trochlearis  nerve,  159 


Trophoblast,  417 
Trypsin,  346 

fate  of,  352 
Trypsinogen,  348 
Tryptophane,  347 
Tubular  glands  of  stomach,  325 
Tubules,  secretion  in  kidney,  406 
Tunica  albuginea,  414 
Turnips  as  food,  321 
Twelfth  nerve,  158 
Tympanic  cavity,  130 
Typhoid  toxin,  392 
Tyrosin,  11,  347,  350 

UMBILICAL  arteries,  420 

veins,  418 

Unilateral  stimulation,  16 
Uiates,  395 
Urea,  315,  369,  372,  396,  424 

of  blood,  201 

effect  of  drugs  on  formation  of, 
370 

source  of,  370 
Ureter,  ligature  of,  405 
Urethra,  407 

Uric  acid,  191,  397,  406,  425 
Urinary  bladder,  4.07 

nerves  of,  407 
Urine,  394  et  seq. 

excretion  of,  407 

method  of  estimating  solids  in, 
394 

micro-organisms  in,  396 

non-urea  nitrogen  in,  396 

of  herbivora,  395 

pigments  of,  402 

secretion  of,  403 
Urabilin,  402 
Urochrome,  402 
Uroerythrin,  402 
Uterus,  414,  415 

centre  for  contractions  of,  420 
Utricle,  133 

VACUOLES,  14 
Vagus,  146,  158 

cardiac  branches,  239  et  seq. 

frog's,  239 

gastric  branches,  264 

effect  on  respiration  of  section, 
291  et  seq. 

effect  on  respiration  of  stimula- 
tion, 292  et  seq. 
Valsalva,  sinus  of,  216 
Valves  of  the  heart,  214  et  seq. 

action  of,  228 

aortic,  217 

auriculo  -  ventricular,     228     (see 
Mitral  and  Tricuspid) 

mitral,  214 

pulmonary,  217 


444 


INDEX 


Valves  of  the  heart,  semilunur,  216 

(see  Aortic  and  Pulmonary) 
Valve,  ileo-caecal,  326 
Vas  deferens,  414 

deferens,  ligature  of,  413 
Vasa  efferentia,  414 
Vaso-constrictor  centre,  265 

centres,  secondary,  266 

nerves,  264 
Vaso-dilator  centre,  l>t'>r, 

nerves,  264 
Vaso-motor  centre  (see  Centre) 

mechanism,  262 

nerves,  263 

nerves,  section  of,  263 
Vegetables,  as  food  (see  Food) 

effect  of  cooking,  323 

green,  321 

Veins,  pressure  in,  225,  246,  258,  269 
et  seq. 

umbilical  (see  Umbilical) 
Vena  cava,  inferior  (foetal),  418 

superior  (foetal),  418 
Venous  pulse,  2.V. 

pulse,  normal.  2.">»; 
Ventilation,  306 
Ventricles  of  the  heart,  211 

contraction  of,  214 

left,  212 

muscular  structure  of,  212.  214 

pressure  in.  'I'll 

right,  214 

Veratrin,  effect  on  muscular  contrac- 
tion, ."it; 

Vermiform  appendix,  326 
Vernon    on    erepsin   in   the   tissues. 

350 

Vesicular  sound,  288 
Vestibule  of  the  ear,  132 
Vestibular  root  of  the  eighth  nerve, 

135,  162 

Vibrations,  light,  119 
Vibratories  (see  Consonants) 
Vieussens,  annulus  of  (sec  Annulus) 
Villi.  chorionic,  417 

of  intestine,  326 
Vision,  102  et  seq.,  164 

binocular,  123 

far  point  of,  111 

field  of,  118 


Vision,  monocular,  106  et  seq. 

near  point  of,  111 

theories  of  colour,  122 
Visual  centre,  128,  181 

sensation,  duration  of,  128 

sensation,  strength  of,  128 
Vital  action  of  endothelium  of  capil- 
laries, 304 

Vital  capacity  of  the  thorax,  286 
Vitreous  humour,  107 
Vocal  cords  (false  and  true),  308 
Voice,  307  et  seq. 

chest,  310 

falsetto,  310 
Volition,  184 
Voluntary  actions,  88,  92 

centres,  184 
Vomiting,  340 

centre,  341 
Vowel  sounds,  311 

WALLERIAN    degeneration   method, 

152 

Waste,  rate  of  (during  fasting),  373 
Water  in  food-stuffs,  312 
Weigert's  method  of  nerve-staining, 

152 

Wheaten  flour  as  food,  321 
Whey  albumin,  319,  336 
Whispered  speech,  310 
White  bread  as  food,  321 

XANTHIN,  320,  383,  398 

X  rays  in   the  examination   of   the 

stomach,  338 
of  the  intestine,  354 

YAWNING,  286 
Yeast,  4,  316 

ZONA  granulosa,  413,  414 

pellucida,  414 

Zuntz  on  excretion  of  CO-,  in  the  dog, 
377 

experiments  on  muscle  work.  (J4 
378 

and  Levy,  on  diet,  374 
Zymin,  7,  317 

secreting  epithelium,  26 
Zymogen,  26 


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UNIVERSITY  OF  CALIFORNIA  LIBRARY 


